Update README; this is mostly to ensure tests of old version are is via Travis.

This was done in relation to #102
From http://docs.travis-ci.com/user/languages/ruby/

In reference to #102

"If you don't specify a version, Travis CI will use MRI 1.9.3 as the default.

When using Rubinius, there's currently an issue with selecting the correct version in RVM in our build environment, but only when specifying rbx as your version. As a workaround, specify rbx-2 instead."
Adjust travis-ci configuration file:
- using `jruby-20mode` has fallen out of fashion it seems `jruby` is the "correct" way at the moment (being 2.1 mode)
- allow for rbx issue related to traivs-ci at the moment.
Adjust RBX versions again.