Update omniauth-salesforce to auth v2 compatibility

Author: melashu

omniauth-salesforce.gemspec

@@ -15,9 +15,10 @@ Gem::Specification.new do |gem|
   gem.version       = OmniAuth::Salesforce::VERSION
   gem.license       = 'MIT'
 
-  gem.add_dependency 'omniauth', '~> 1.0'
-  gem.add_dependency 'omniauth-oauth2', '>= 1.5.0'
+  gem.add_dependency 'omniauth', '~> 2.0'
+  gem.add_dependency 'omniauth-oauth2', '~> 1.2'
   gem.required_ruby_version = '>= 2.1.0'
+  gem.add_dependency 'rack', '>= 3.0.0', '< 4'
   gem.add_development_dependency 'rack-test'
   gem.add_development_dependency 'rspec', '~> 2.7'
   gem.add_development_dependency 'simplecov'