Test behaviour when idn-ruby is installed

[dentarg]

Addressable favours the use of libidn (via the idn-ruby gem) to it's pure-Ruby IDNA implementation. twingly-url is built with the expectation that the pure-Ruby implementation is used.

How does this affect twingly-url, if idn-ruby is present on the sytem (or in the Gemfile when using Bundler)?

From #120

[dentarg]

Relevant: https://docs.travis-ci.com/user/languages/ruby/#Testing-against-multiple-versions-of-dependencies