Travis could not find rbx-19mode,changed to latest

The rubinius build on travis was referring to rbx-19mode, but that's no longer available on rvm, and that particular build was always erroring out. Changed it to always take the latest release of rubinius instead
This actually needs to be "rbx-2" now. Would you mind amending this commit?  See https://github.com/metricfu/metric_fu/commit/5352124bc5a0c831fb41a547896488c9cf2b43ab#diff-354f30a63fb0907d4ad57269548329e3