Remove development dependencies from actioncable.gemspec

None of the other components use them, so we should be consistent.

actioncable/actioncable.gemspec

@@ -22,13 +22,4 @@
 
   s.add_dependency 'nio4r',            '~> 1.2'
   s.add_dependency 'websocket-driver', '~> 0.6.1'
-
-  s.add_development_dependency 'coffee-script',        '~> 2.4.1'
-  s.add_development_dependency 'coffee-script-source', '~> 1.10.0'
-  s.add_development_dependency 'em-hiredis',           '~> 0.3.0'
-  s.add_development_dependency 'mocha'
-  s.add_development_dependency 'pg'
-  s.add_development_dependency 'puma'
-  s.add_development_dependency 'redis',                '~> 3.0'
-  s.add_development_dependency 'sprockets',            '~> 3.5.2'
 end