Don't require client and channel

for Google Premier. No longer required, according to this page:
https://developers.google.com/maps/premium/overview

Fixes #1268

lib/geocoder/lookups/google_premier.rb

@@ -11,7 +11,7 @@ module Geocoder::Lookup
     end
 
     def required_api_key_parts
-      ["private key", "client", "channel"]
+      ["private key"]
     end
 
     def query_url(query)