Remove spree_auth_devise dependency

Gemfile

@@ -1,6 +1,3 @@
 source 'http://rubygems.org'
 
-# TODO: Move this requirement to gemspec once spree_auth_devise is released as a gem
-gem 'spree_auth_devise', :git => 'git://github.com/spree/spree_auth_devise.git'
-
 gemspec

lib/spree_paypal_express.rb

@@ -1,3 +1,2 @@
 require 'spree_core'
-require 'spree_auth_devise'
 require 'spree_paypal_express/engine'