Travis CI tweaks

Some minor Travis CI tweaks to improve performance/speed of tests.  We seem to still be having some randomly "Killed" builds in Travis CI, which I suspect may be memory related.

The following minor tweaks to `.travis.yml` are included in this PR:
* Speed tweak: Skip the 'mvn install' process, which Travis uses to download all Maven dependencies. It's really not needed. We can just let Travis download these during the build/test.
* Performance tweak: Set MAVEN_OPTS to give Maven 2GB of memory to work with. By default, on Travis CI, this seems to be set to 800MB in JAVA_OPTS by default (or at least that's what Travis is telling me when I tell it to run 'echo $JAVA_OPTS' as part of the build)
* Finally, I removed the `cache` settings. On further review, they are only offered for private repos in the paid (travis-ci.com) plans. See http://docs.travis-ci.com/user/caching/

Assuming Travis CI doesn't complain about this PR, I'm going to merge this immediately. I don't think any of this is controversial, and I'm hoping the tweak to MAVEN_OPTS will help avoid some of the "Killed" builds.