Remove JRuby 1.7 and Rubinius from the test matrix
Remove JRuby 1.7 and Rubinius from the test matrix

Since Travis made Xenial the default build environment, JRuby 1.7 no
longer runs, and Rubinius can't even be installed:

https://travis-ci.org/cypriss/mutations/jobs/589829405
https://travis-ci.org/cypriss/mutations/jobs/589829414

Given that these implementations are rarely used, I'm removing them from
the test matrix to get the build green again.

cypriss/mutations