Required version and provider need to be added to Generator DSL

[christianblunden]

The two helper methods on Generator to create the required version and providers config
https://github.com/uswitch/terrafying/blob/master/lib/terrafying/generator.rb#L202
https://github.com/uswitch/terrafying/blob/master/lib/terrafying/generator.rb#L210

need to be added to the DSL,
https://github.com/uswitch/terrafying/blob/master/lib/terrafying/generator.rb#L359

otherwise they just get caught with the method missing and get exposed as resources
https://github.com/uswitch/terrafying/blob/master/lib/terrafying/generator.rb#L343