Fix #1378. Need to always have a slug to replace on.

Author: ericholscher

readthedocs/builds/models.py

@@ -215,6 +215,8 @@ def get_vcs_slug(self):
                 slug = self.project.vcs_repo().fallback_branch
         elif self.slug == STABLE:
             return self.identifier
+        else:
+            slug = self.slug
         # https://github.com/rtfd/readthedocs.org/issues/561
         # version identifiers with / characters in branch name need to un-slugify
         # the branch name for remote links to work