Add an initial travis CI configuration file.

FYI, I've been experimenting locally w/ getting this working too, and have it working for the no_aop build as well, see: https://github.com/sameb/guice/blob/master/.travis.yml  ... maybe merge some of that w/ this?  (you know the maven args better than I do.)  ... latest builds with that are: https://travis-ci.org/sameb/guice/builds/29466236 -- there's some flaky assistedinject tests i need to fix.  (also: i needed to turn off javadoc generation because it broke jdk8.)