Rbx 2 travis

* change travis tests from rubinius 2.1.1 to `rbx-2.2` which uses latest rubinius-2.2.x version
* improve `with_unicorn` parameter structure

Rubinius tests could now potentially run for every version from 2.2.0-2.2.7 and 2.1.1. To avoid testing for so many releases, another idea is to use `rbx-2.2` and get tests for the latest minor release. As it stands, 2.1.1 doesn't seem to run correctly on travis due to missing bundler. The 2.2.x branch fixed this and packages bundler by default, including additional rubysl gems.