Keep the API docs building

docs/en_us/platform_api/requirements.txt

@@ -13,3 +13,4 @@ stevedore==0.14.1
 djangorestframework==3.2.3
 PyYAML>=3.10
 mock
+django-model-utils==2.3.1

docs/en_us/platform_api/source/conf.py

@@ -52,6 +52,7 @@ MOCK_MODULES = [
     'xmodule.modulestore',
     'xmodule.modulestore.exceptions',
     'xmodule.modulestore.django',
+    'xmodule.fields',
     'courseware.models',
     'milestones',
     'milestones.api',
@@ -113,9 +114,6 @@ MOCK_MODULES = [
     'xmodule.modulestore.mongo.base',
     'edxval',
     'edxval.api',
-    'model_utils',
-    'model_utils.models',
-    'model_utils.managers',
     'certificates',
     'certificates.models',
     'certificates.models.GeneratedCertificate',