Revert "Run tests on upcoming JRuby 9000"
Revert "Run tests on upcoming JRuby 9000"

It seems that defining `jruby` in .travis.yml already runs
JRuby 9000.

See https://travis-ci.org/rom-rb/rom/jobs/45789302#L102

This reverts commit dfd264fd2b8536d4e6c849d01837eca62af763cd.

denyago/rom,rom-rb/rom,cored/rom,pxlpnk/rom,pdswan/rom,vrish88/rom,Snuff/rom,kwando/rom,dekz/rom,endash/rom,jeremyf/rom,pvcarrera/rom,rom-rb/rom,dcarral/rom,rom-rb/rom