Issue with port option in 2.4.0 version
Hi,
I update to the 2.4 today and i got issue with docker-compose when i try to add the following line to my configuration file (docker-compose.yml) : 
`ports:
            - "127.0.0.1:9292:9090"`

I got the following error:

`
ERROR: for ContainerName  expected string or buffer
Traceback (most recent call last):
  File "/usr/local/bin/docker-compose", line 11, in <module>
    sys.exit(main())
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/main.py", line 68, in main
    command()
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/main.py", line 118, in perform_command
    handler(command, command_options)
  File "/usr/local/lib/python2.7/dist-packages/compose/cli/main.py", line 926, in up
    scale_override=parse_scale_args(options['--scale']),
  File "/usr/local/lib/python2.7/dist-packages/compose/project.py", line 424, in up
    get_deps
  File "/usr/local/lib/python2.7/dist-packages/compose/parallel.py", line 69, in parallel_execute
    raise error_to_reraise
TypeError: expected string or buffer
`

I have no issue when i downgrade again to the 2.3 version of the package

To reproduce the issue, i use the following configuration ( it doesn't seem to depend on the image):
```
version: '2'

services :
    ContainerName:
        image: bae2d441e03a
        ports:
            - "127.0.0.1:9292:9090"
```

I run on Ubuntu 14.04.5 LTS with the following package:
```
docker==2.4.0
docker-compose==1.14.0
docker-pycreds==0.2.1
dockerpty==0.4.1
Python 2.7.6
Client:
 Version:      17.05.0-ce
 API version:  1.29
 Go version:   go1.7.5
 Git commit:   89658be
 Built:        Thu May  4 22:06:06 2017
 OS/Arch:      linux/amd64

Server:
 Version:      17.05.0-ce
 API version:  1.29 (minimum version 1.12)
 Go version:   go1.7.5
 Git commit:   89658be
 Built:        Thu May  4 22:06:06 2017
 OS/Arch:      linux/amd64
 Experimental: false
```
diff --git a/docker/utils/ports.py b/docker/utils/ports.py
index 57332dee..8f713c72 100644
--- a/docker/utils/ports.py
+++ b/docker/utils/ports.py
@@ -54,6 +54,7 @@ def port_range(start, end, proto, randomly_available_port=False):
 
 
 def split_port(port):
+    port = str(port)
     match = PORT_SPEC.match(port)
     if match is None:
         _raise_invalid_port(port)
diff --git a/tests/unit/utils_test.py b/tests/unit/utils_test.py
index c25881d1..a2d463d7 100644
--- a/tests/unit/utils_test.py
+++ b/tests/unit/utils_test.py
@@ -587,6 +587,9 @@ class PortsTest(unittest.TestCase):
     def test_split_port_empty_string(self):
         self.assertRaises(ValueError, lambda: split_port(""))
 
+    def test_split_port_non_string(self):
+        assert split_port(1243) == (['1243'], None)
+
     def test_build_port_bindings_with_one_port(self):
         port_bindings = build_port_bindings(["127.0.0.1:1000:1000"])
         self.assertEqual(port_bindings["1000"], [("127.0.0.1", "1000")])
['tests/unit/utils_test.py', 'docker/utils/ports.py']