Bump Rugged to 0.28.1

This resolves a minor link issue in libgit2 with deprecated functions:
https://github.com/libgit2/libgit2/releases

This coincides with the Gitaly bump:
https://gitlab.com/gitlab-org/gitaly/merge_requests/1154

Gemfile.lock

@@ -784,7 +784,7 @@ GEM
     rubyntlm (0.6.2)
     rubypants (0.2.0)
     rubyzip (1.2.2)
-    rugged (0.28.0)
+    rugged (0.28.1)
     safe_yaml (1.0.4)
     sanitize (4.6.6)
       crass (~> 1.0.2)