Register :msgpack as a renderable content type

Enable a Rails server to respond to 'application/msgpack' requests with
a renderer that calls `to_msgpack`.

This commit has no tests and may not handle render options in an optimal
way, so it may be desirable to view it as a working proof-of-concept. It
does appear to work, though.
Use the latest minitest
Clean up registration of mime type and renderer
Test MRI only
Merge branch 'NuffieProductions-add_renderer'
Fix minitest and test/unit compatibility issue, see seattlerb/minitest#358
Mix into ActiveRecord only when defined
Add support for serializers in Mongoid