Update shoulda to 3.5.0

This release is compatible with Rubinius, 3.3.2 just seems to crash.

Also use an older version of shoulda-matchers that works with MRI <=
1.9.2
Disable coverage for Rubinius

It's too slow for now: https://github.com/rubinius/rubinius/issues/2730
Add rubysl for rubinius
Add Rubinius 2.2.1, 2.2.0 and 2.1.1 to the Travis build matrix