Use mediawiki_api-wikidata gem for interaction with Wikidata API

Got rid of the ugly wikibase_api_module and use mediawiki_api-wikidata ruby gem instead. More information about the gem can be found here: https://rubygems.org/gems/mediawiki_api-wikidata

The code dealing with entity data is still not nice though. To be able to deal with it better we would require an implementation of the Wikibase DataModel in Ruby. Would be nice to have.
@adrianlang Good catch! Fixed in newest commit.