Updated oath2 dependency 

Updated the oauth2 dependance to be ~> 0.9.1. The reason for the change is people who use SalesForce's bulk API might also use the restforce gem which is used for SalesForce's streaming API. Restforce gem requires faraday ~> 0.8.4 where the current version of oauth2 4.1 uses faraday ~> 0.6.1.  

I have tested oauth2 0.9.1 with my app and can still authentication and use the bulk API without issue.