Add post_install note about removing oauth/oauth2 dependencis

#780

sorcery.gemspec

@@ -10,6 +10,8 @@ Gem::Specification.new do |s|
   s.description = "Provides common authentication needs such as signing in/out, activating by email and resetting password."
   s.summary = "Magical authentication for Rails 3 & 4 applications"
   s.homepage = "http://github.com/NoamB/sorcery"
+  s.post_install_message = "As of version 1.0 oauth/oauth2 won't be automatically bundled\n"
+  s.post_install_message += "you need to add those dependencies to your Gemfile"
 
   s.files         = `git ls-files`.split($/)
   s.require_paths = ["lib"]