Use Travis to test against more rubies

Travis can test with Ruby implementations besides 1.9.3. This commit expands the matrix into also including JRuby in 1.9 mode, Rubinius in 1.9 mode, jruby-head, and ruby-head, all of which currently pass.

This doesn't catch the issue in #89 (see [this test build](https://travis-ci.org/willglynn/activerecord-postgres-hstore/builds/5819636)), suggesting some gaps in test coverage, but at least this way *something* gets run under JRuby before each merge.