Locked the omniauth-oauth2 version to 1.3.1

omniauth/omniauth-oauth2#70 introduced a
breaking change for a number of oauth providers.  Right now the
recommended fix is to lock to 1.3.1

Here's the relevant doorkeeper discussion.
doorkeeper-gem/doorkeeper#737

omniauth-mlh.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |spec|
  spec.require_paths = ["lib"]
 
  spec.add_dependency 'omniauth', '~> 1.0'
- spec.add_dependency 'omniauth-oauth2', '>= 1.1.1', '< 2.0'
+ spec.add_dependency 'omniauth-oauth2', '~> 1.3.1'
 
  spec.add_development_dependency 'rspec', '~> 2.7'
  spec.add_development_dependency 'rack-test'