Ensured get_version returns a native string.

Returning unicode triggers a bug in Python 2.7:
http://bugs.python.org/issue11638

This problem was introduced in 4a103086 (unicode_literals).

django/utils/version.py

@@ -30,7 +30,7 @@ def get_version(version=None):
         mapping = {'alpha': 'a', 'beta': 'b', 'rc': 'c'}
         sub = mapping[version[3]] + str(version[4])
 
-    return main + sub
+    return str(main + sub)
 
 def get_git_changeset():
     """Returns a numeric identifier of the latest git changeset.