Update oauth2 gem to 1.4

Author: dennisvdvliet

omniauth-oauth2.gemspec

@@ -3,7 +3,7 @@ $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)
 require "omniauth-oauth2/version"
 
 Gem::Specification.new do |gem|
-  gem.add_dependency "oauth2",     "~> 1.1"
+  gem.add_dependency "oauth2",     "~> 1.4"
   gem.add_dependency "omniauth",   "~> 1.9"
 
   gem.add_development_dependency "bundler", "~> 1.0"