Added Rails4 compatibility

A few things have moved around in recent versions of minitest-rails... see https://github.com/blowmage/minitest-rails/wiki/Upgrading-to-0.9 for details.

Also,

Recent versions of shoulda-matchers (apparently) have removed the
respond_with_content_type matcher.  I only guess so because I can't
find it in the current version.  Therefore I've replaced those tests
with (imho) suitable replacement matchers.

*I doubt* you'll want to merge this pull request with master... but I wanted to show you this work in case you're ready to add a rails4 branch.