Load the pure-Ruby IDNA implementation from Addressable
Load the pure-Ruby IDNA implementation from Addressable

Since #113, twingly-url can only be used with the pure-Ruby
implementation in Addressable.

If idn-ruby (libidn wrapper gem) is installed, Addressable will not load
the pure-Ruby implementation, which breaks twingly-url.

Close #119

twingly/twingly-url