Drop TravisCI support for Ruby 1.9.2

* There were issues with activesupport 4 and Ruby 1.9.2.
  https://travis-ci.org/thoughtbot/clearance/jobs/9306920
* Ignore Gemfile.lock. This is standard when generating a new gem with
  `bundle gem`.
  http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/