Import error on pip install
When installing in a new environment (say in docker on deploy), the pip install will fail with `ImportError: No module named 'django'`.

This happens because setup.py does a `from wagtailmenus import __version__` which loads `__init__.py` which in itself imports django. It should not assume, however, django is already built and installed during a `pip install -r requirements.txt`.
diff --git a/wagtailmenus/__init__.py b/wagtailmenus/__init__.py
index 22af1b5..6eb9631 100644
--- a/wagtailmenus/__init__.py
+++ b/wagtailmenus/__init__.py
@@ -1,5 +1,4 @@
 from __future__ import absolute_import, unicode_literals
-from django.core.exceptions import ImproperlyConfigured
 from .app_settings import Settings
 
 __version__ = '2.1.0'
@@ -35,6 +34,8 @@ def get_main_menu_model():
     if no custom model is defined.
     """
     from django.apps import apps
+    from django.core.exceptions import ImproperlyConfigured
+
     model_string = get_main_menu_model_string()
     try:
         return apps.get_model(model_string)
@@ -57,6 +58,8 @@ def get_flat_menu_model():
     if no custom model is defined.
     """
     from django.apps import apps
+    from django.core.exceptions import ImproperlyConfigured
+
     model_string = get_flat_menu_model_string()
     try:
         return apps.get_model(model_string)
['wagtailmenus/__init__.py']