fix(widgets): use standard way to include static assests

While using `ManifestStaticFilesStorage` there seems to be some issues
around use of `staticfilestore.url` in `widgets.py` while running
`collectstatic` command.

It raises the following `ValueError`:

```
ValueError: The file 'versatileimagefield/css/versatileimagefield.css' could not be found with <whitenoise.django.GzipManifestStaticFilesStorage
```

This issue is not seen with other widgets that use normal method of
including Form Assets[1]. Things works fine, after this change.

[1] https://docs.djangoproject.com/en/1.9/topics/forms/media/#assets-as-a-static-definition

versatileimagefield/widgets.py

@@ -4,7 +4,6 @@
     CheckboxInput, ClearableFileInput,
     HiddenInput, MultiWidget, Select
 )
-from django.contrib.staticfiles.storage import staticfiles_storage
 from django.utils.encoding import force_text
 from django.utils.html import conditional_escape, format_html
 from django.utils.safestring import mark_safe
@@ -185,17 +184,9 @@ def __init__(self, widgets=None, attrs=None, image_preview_template=None):
 
     class Media:
         css = {
-            'all': (
-                staticfiles_storage.url(
-                    'versatileimagefield/css/versatileimagefield.css'
-                ),
-            ),
+            'all': ('versatileimagefield/css/versatileimagefield.css',),
         }
-        js = (
-            staticfiles_storage.url(
-                'versatileimagefield/js/versatileimagefield.js'
-            ),
-        )
+        js = ('versatileimagefield/js/versatileimagefield.js',)
 
     def render(self, name, value, attrs=None):
         rendered = super(VersatileImagePPOIClickWidget, self).render(
@@ -213,12 +204,7 @@ class SizedImageCenterpointClickDjangoAdminWidget(
 
     class Media:
         css = {
-            'all': (
-                staticfiles_storage.url(
-                    'versatileimagefield/css/versatileimagefield-'
-                    'djangoadmin.css'
-                ),
-            ),
+            'all': ('versatileimagefield/css/versatileimagefield-djangoadmin.css',),
         }
 
 
@@ -256,10 +242,5 @@ class SizedImageCenterpointClickBootstrap3Widget(
 
     class Media:
         css = {
-            'all': (
-                staticfiles_storage.url(
-                    'versatileimagefield/css/versatileimagefield-'
-                    'bootstrap3.css'
-                ),
-            ),
+            'all': ('versatileimagefield/css/versatileimagefield-bootstrap3.css',),
         }