Reduce the number of jobs run in each Travis build

There's no need to have two different JRUBY_OPTS for *every* single
ruby, only JRuby.
Don't test on 1.8.7

1.8.7 has been deprecated, no real need to support it anymore.
Merge pull request #12 from henrikhodne/hh-reduce-number-of-travis-jobs

Reduce the number of jobs run in each Travis build