Improvements for Travis CI builds

I couldn't bear watching the red builds reappear in my sidebar on Travis CI :)

This is a work in progress, bringing better support to run tests on both the current Riak (1.4.2) and Riak 2.0.0pre releases.

Some specs are tagged with a Riak version to allow excluding them.

Currently there are two failing tests on 1.4.2, and quite a few on 2.0.0.

Any insight regarding the error messages?

I've removed a few Ruby releases temporarily, planning on bringing them back once the build is running again properly.