Fix this._deleteBearerToken is undefined error

This context wasn't properly set in the catch block for refresh errors, so the token was never deleted.

lib/oauth.js

@@ -277,7 +277,7 @@ module.exports = new Model({
         console.log(error);
         this._deleteBearerToken();
         return null;
-      });
+      }.bind(this));
   },
 
   _saveRedirectUri: function(redirectUri) {