Remove unneeded dependencies for Rubinius
Update the Travis configuration file

Let's avoid running the test suite against the current development
version of Rails (5.0) for rubies older than 2.2 as the latter now
requires it as the minimum Ruby version.

Also fix the name of the Rubinius' binary and add 2.2 in the matrix.
Test against the 4-2-stable branch of Rails as well
Merge pull request #65 from robin850/travis-build

Improve the Travis build