Drop TravisCI support for Ruby 1.9.2

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