vfx.scroll giving TypeError: slice indices must be integers or None or have an __index__ method
I run:
```clip = ImageClip("panorama.jpg")
new_clip = vfx.scroll(clip, w=1000, x_speed=5)
new_clip = new_clip.set_duration(float(clip.w - 1000) / 5)
new_clip.write_videofile("pano.mp4", fps=24)
```
as recommended by Zulko in #240, but it gives the following error:

```Traceback (most recent call last):
  File "/Users/tomburrows/Dropbox/Python Programming/QF_Editor/panorama_scroll.py", line 8, in <module>
    new_clip.write_videofile("pano.mp4")
  File "<decorator-gen-51>", line 2, in write_videofile
  File "/anaconda/lib/python3.5/site-packages/moviepy/decorators.py", line 54, in requires_duration
    return f(clip, *a, **k)
  File "<decorator-gen-50>", line 2, in write_videofile
  File "/anaconda/lib/python3.5/site-packages/moviepy/decorators.py", line 137, in use_clip_fps_by_default
    return f(clip, *new_a, **new_kw)
  File "<decorator-gen-49>", line 2, in write_videofile
  File "/anaconda/lib/python3.5/site-packages/moviepy/decorators.py", line 22, in convert_masks_to_RGB
    return f(clip, *a, **k)
  File "/anaconda/lib/python3.5/site-packages/moviepy/video/VideoClip.py", line 345, in write_videofile
    progress_bar=progress_bar)
  File "/anaconda/lib/python3.5/site-packages/moviepy/video/io/ffmpeg_writer.py", line 209, in ffmpeg_write_video
    fps=fps, dtype="uint8"):
  File "/anaconda/lib/python3.5/site-packages/tqdm/_tqdm.py", line 833, in __iter__
    for obj in iterable:
  File "/anaconda/lib/python3.5/site-packages/moviepy/Clip.py", line 475, in generator
    frame = self.get_frame(t)
  File "<decorator-gen-14>", line 2, in get_frame
  File "/anaconda/lib/python3.5/site-packages/moviepy/decorators.py", line 89, in wrapper
    return f(*new_a, **new_kw)
  File "/anaconda/lib/python3.5/site-packages/moviepy/Clip.py", line 95, in get_frame
    return self.make_frame(t)
  File "/anaconda/lib/python3.5/site-packages/moviepy/Clip.py", line 136, in <lambda>
    newclip = self.set_make_frame(lambda t: fun(self.get_frame, t))
  File "/anaconda/lib/python3.5/site-packages/moviepy/video/fx/scroll.py", line 17, in f
    return gf(t)[y:y+h, x:x+w]
TypeError: slice indices must be integers or None or have an __index__ method
```
Doing some research, this is because `x` is  0 when you first call it, but then when it writes the video to a file, it iterates for each frame, and the second time, x is 1.0. This is a float, not an int so it fails.
A solution that worked for me is to change lines 14 and 15 of `scroll.py` to change the float into a int.
Can anyone repro this, and shall we make the suggested changes?
diff --git a/moviepy/video/fx/scroll.py b/moviepy/video/fx/scroll.py
index 59a285c..34f4f40 100644
--- a/moviepy/video/fx/scroll.py
+++ b/moviepy/video/fx/scroll.py
@@ -1,5 +1,3 @@
-import numpy as np
-
 def scroll(clip, h=None, w=None, x_speed=0, y_speed=0,
            x_start=0, y_start=0, apply_to="mask"):
     """ Scrolls horizontally or vertically a clip, e.g. to make end
@@ -11,8 +9,8 @@ def scroll(clip, h=None, w=None, x_speed=0, y_speed=0,
     ymax = clip.h-h-1
 
     def f(gf,t):
-        x = max(0, min(xmax, x_start+ np.round(x_speed*t)))
-        y = max(0, min(ymax, y_start+ np.round(y_speed*t)))
+        x = int(max(0, min(xmax, x_start+ round(x_speed*t))))
+        y = int(max(0, min(ymax, y_start+ round(y_speed*t))))
         return gf(t)[y:y+h, x:x+w]
     
     return clip.fl(f, apply_to = apply_to)
diff --git a/tests/download_media.py b/tests/download_media.py
index 5d735ba..0cc75d2 100644
--- a/tests/download_media.py
+++ b/tests/download_media.py
@@ -48,4 +48,6 @@ def download():
 
     download_url("https://raw.githubusercontent.com/earney/moviepy_media/master/tests/misc/traj.txt",
                  "media/traj.txt")
-    
+
+    download_url("https://github.com/earney/moviepy_media/raw/master/tests/images/vacation_2017.jpg",
+                  "media/vacation_2017.jpg")
\\ No newline at end of file
diff --git a/tests/test_PR.py b/tests/test_PR.py
index 790c65c..aba1c3e 100644
--- a/tests/test_PR.py
+++ b/tests/test_PR.py
@@ -89,9 +89,19 @@ def test_PR_515():
     clip = VideoFileClip("media/fire2.mp4", fps_source='fps')
     assert clip.fps == 10.51
 
+    
+def test_PR_528():
+    clip = ImageClip("media/vacation_2017.jpg")
+    new_clip = vfx.scroll(clip, w=1000, x_speed=50)
+    new_clip = new_clip.set_duration(20)
+    new_clip.fps = 24
+    new_clip.write_videofile(os.path.join(TMP_DIR, "pano.mp4"))
+    
+
 def test_PR_529():
     video_clip = VideoFileClip("media/fire2.mp4")
     assert video_clip.rotation ==180
 
+
 if __name__ == '__main__':
    pytest.main()
['tests/test_PR.py', 'tests/download_media.py', 'moviepy/video/fx/scroll.py']