Add canonical_identifier and canonical_zone methods to Timezone.

Closes #16.
Use assert_nothing_raised.
Test using the latest Ruby 2.1 release.

Ruby is now using semantic versioning:
https://www.ruby-lang.org/en/news/2013/12/21/semantic-versioning-after-2-1-0/

Travis CI supports referencing ruby-2.1 following this update:
http://blog.travis-ci.com/2014-04-28-upcoming-build-environment-updates/