Workaround ActiveSupport::Concern error

See laserlemon#111. This issue
seems to be the gem defines 2 Version::VersionMethods modules, one users.rb
and one in version_tagging.rb, each with an `included` block.

As quick proof of concept rename the module in VersionTagging (since we
don't use that feature) to `VersionTaggingMethods` and update the
`class_eval` block to reflect the change.

A better approach might be to namesoace each `VersionMethods` module to
the parent module, e.g. `VestalVersions::VersionTagging::VersionMethods`
and `VestalVersions::User::VersionMethods`

lib/vestal_versions/version_tagging.rb

@@ -23,7 +23,7 @@ def tag_version(tag)
   end
 
   # Instance methods included into VestalVersions::Version to enable version tagging.
-  module VersionMethods
+  module VersionTaggingMethods
     extend ActiveSupport::Concern
 
     included do
@@ -46,6 +46,6 @@ def validate_tags?
       tagged? && tag != 'deleted'
     end
 
-    Version.class_eval{ include VersionMethods }
+    Version.class_eval{ include VersionTaggingMethods }
   end
 end