Improve OmniAuth version check to allow anything from 1.0 forward

This should enable people to try OmniAuth 2 currently in pre-release.

lib/devise/omniauth.rb

@@ -8,8 +8,8 @@ rescue LoadError
   raise
 end
 
-unless OmniAuth::VERSION =~ /^1\./
-  raise "You are using an old OmniAuth version, please ensure you have 1.0.0.pr2 version or later installed."
+if Gem::Version.new(OmniAuth::VERSION) < Gem::Version.new('1.0.0')
+  raise "You are using an old OmniAuth version, please ensure you have 1.0.0 version or later installed."
 end
 
 # Clean up the default path_prefix. It will be automatically set by Devise.