Enable Travis' bundle caching

It should speed up the builds significantly. See
http://about.travis-ci.org/docs/user/caching/#Bundler for more details.
Add Rubinius 2.2.1, 2.2.0 and 2.1.1 to the Travis build matrix
Add rubysl and rubinius-coverage for rubinius
Use an older version of shoulda-matchers that works with MRI <= 1.9.2
Merge branch 'master' into rubinius

Conflicts:
	.travis.yml