Drop jruby 1.6.8 from travis.yml

As of this time jruby 1.6.8 is broken in RVM, as RVM installs an
incompatible version of jruby-openssl.
Use rbx-2.1.1 in travis.yml
Use rbx-19mode but allow its failure in Travis
Update to rjack-tarpit 2.1.0; resolves TravisCI issue with SSL
Merge branch 'dev' into thread-safe