Workaround for Travis/RVM JRuby issue.

Specify a version for JRuby as per:
https://github.com/travis-ci/travis-ci/issues/3067

Using version 1.7.18 would require uninstalling and reinstalling JRuby (see
thread above) which would be done for every entry in the build matrix, even the
non-JRuby ones. So use 1.7.17 for now.