Make the specs pass with more versions of Ruby

`#to_yaml` has different behaviour between Ruby versions. The current
specs pass with some Ruby versions and fail with others. For example:

    # 1.9.3
    {:foo => "~bar"}.to_yaml # => "---\n:foo: ~bar\n"

    # 2.1.0
    {:foo => "~bar"}.to_yaml # => "---\n:foo: \"~bar\"\n"
Add MRI 1.9.2 and 2.1.0 to .travis.yml
Merge pull request #94 from pearkes/travis-rubies

Add MRI 1.9.2 and 2.1.0 to .travis.yml