Remove ym4r and add geocoder.

I was having issues using any queries that required an address field. The stacktrace was showing an issue with ym4r. Since ym4r doesn't look like its being maintained, I replaced the gem with geocoder. All tests now pass and the address queries work fine.  

     OpenURI::HTTPError:
       403 Forbidden
     # ./lib/sunlight/district.rb:26:in `get'
     # ./lib/sunlight/legislator.rb:71:in `all_for'
     # ./spec/legislator_spec.rb:91:in `block (3 levels) in <top (required)>'