bump omniauth-oauth2 dependency version (CSRF issue)

http://www.rubysec.com/advisories/CVE-2012-6134/
```
CVE-2012-6134 in omniauth-oauth2 
Cross-site request forgery (CSRF) vulnerability in the omniauth-oauth2 gem 1.1.1 and earlier for Ruby allows remote attackers to hijack the authentication of users for requests that modify session state.
```