exclude rbx for now, instead add 2.0.0
Don't test on 1.9.2, head and rbx-18mode for now
Don't test on 1.8.7
1.8.7 has been deprecated, no real need to support it anymore.
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.
Test against 2 JRuby modes
remove ruby-head from .travis.yml matrix
add 1.9.3 and jruby
add a .travis.yml with some rubies