Update tests to use MiniTest
Remove Gemfile.lock from source control

`Gemfile.lock` should not be commited to source control when working on
a gem:
<http://yehudakatz.com/2010/12/16/clarifying-the-roles-of-the-gemspec-and-gemfile/>
Add UTF-8 magic comment for Ruby 1.9.3
Add Travis CI status image to README
Change Travis CI branch to master