Depend on Active Support

Gravatar depends on Active Support, but it's only listed as a development dependency.

	/Users/jgorset/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/gravatar-ultimate-1.0.3/lib/gravatar.rb:1:in `require':
	 cannot load such file -- active_support (LoadError)
	        from /Users/jgorset/.rbenv/versions/1.9.3-p194/lib/ruby/gems/1.9.1/gems/gravatar-ultimate-1.0.3/lib/gravatar.rb:1:
	in `<top (required)>'
	        ...