Generate coverage reports only when using MRI

Rubinius generates falsy coverage statistics, JRuby is also not
officially supported by SimpleCov.
See: https://github.com/lemurheavy/coveralls-public/issues/144
Patch UUID as a reference to Youyouaidi::UUID class

Now, e.g. `UUID.valid?` can be used as a shorthand for
`Youyouaidi::UUID.valid?`.
Add more detailed description of .valid? to README.md