Travis: Modern CI matrix, now green

[olleolleolle]

This PR updates the CI matrix to the latest generally available Ruby versions.

• drop rbx (uninstallable)
• specify exact JRuby version (and use JRUBY_OPTS=--debug to get code coverage numbers)
• use current Ruby versions
• configured Travis to use dist: trusty (instead of the default Precise)

---

Issues encountered:

• too-long install time for Travis (was Precise missing the haveged apt package?) - this was fixed by switching to dist: trusty on Travis

[coveralls]

Coverage decreased (-0.04%) to 97.6% when pulling 6668b63 on olleolleolle:patch-2 into 9a6a6e3 on kaize:master.

[coveralls]

Coverage decreased (-0.04%) to 97.6% when pulling 6668b63 on olleolleolle:patch-2 into 9a6a6e3 on kaize:master.

[coveralls]

Coverage decreased (-0.04%) to 97.6% when pulling f17073d on olleolleolle:patch-2 into 9a6a6e3 on kaize:master.

[coveralls]

Coverage decreased (-0.04%) to 97.6% when pulling b2304a4 on olleolleolle:patch-2 into 9a6a6e3 on kaize:master.

[coveralls]

Coverage decreased (-0.04%) to 97.6% when pulling b2304a4 on olleolleolle:patch-2 into 9a6a6e3 on kaize:master.

[olleolleolle]

Well, that's as good as this grease-up goes.

JRuby outputs coverage data, too, now.