Remove jruby and 1.9.3 from build matrix

The `mutant` gem [specifies Ruby >= 2.0.0](https://github.com/mbj/mutant/blob/75164ba31a9129fc16d27d7f4a3e741a90da4a12/mutant.gemspec#L24) which is causing [the build to fail](https://travis-ci.org/mgsnova/feature/builds/43925223).