Add check to prevent build of docs with bad version

.travis.yml

@@ -16,7 +16,7 @@ jobs:
       dist: xenial
       sudo: required
     - python: '3.6'
-    env: TOXENV=docs
+      env: TOXENV=docs
     - python: '3.6'
       env: TOXENV=djmaster
     - python: '3.6'

docs/conf.py

@@ -48,6 +48,11 @@ author = 'Cristi V.'
 
 # The full version, including alpha/beta/rc tags.
 release = get_distribution('drf_yasg').version
+if 'noscm' in release:
+    raise AssertionError('Invalid package version string: %s. \n'
+                         'The documentation must be built with drf_yasg installed from a distribution package, '
+                         'which must have been built with a proper version number (i.e. from a full source checkout).'
+                         % (release,))
 
 # The short X.Y.Z version.
 version = '.'.join(release.split('.')[:3])