Make Devise find the strategy class automatically

Since we're already setting `wordpress_oauth2` using the `option :name`
command in the `OmniAuth::Strategies::Wordpress` class, Devise can
automatically find the strategy. However, it wasn't working because we
were passing a string instead of a symbol.

lib/omni_auth/strategies/wordpress.rb

@@ -5,7 +5,7 @@ require "omniauth-oauth2"
 module OmniAuth
   module Strategies
     class Wordpress < OmniAuth::Strategies::OAuth2
-      option :name, "wordpress_oauth2"
+      option :name, :wordpress_oauth2
 
       option :client_options, {}