Use activesupport version compatible with Ruby

Rails 4 is out, so in order to make the Gem working on 1.9.2, we
must specify that ActiveSupport ~> 3 should be used if ruby < 1.9.3
Support Ruby 1.9.2 by using the ActiveSupport ~> 3
Don't run Travis CI on Rubinius until stable

Rubinius 2.0.0 is still RC and is not stable, so might as well
not run Travis until it works.
(For instance, see https://travis-ci.org/topspin/id3_tags/jobs/8439852)
Refactor: decrease code complexity of M4A Class

Split long functions into more shorter ones