Refactor image_url method

* Stop passing `options` to the image_url method since `options` is an
instance method.

* Handle image urls that do not end in `photo.jpg`. In that case, the
original image is returned instead. A test case was added for this situation.
Lock down the omniauth-oauth2 gem version
Merge pull request #104 from guilhermesimoes/refactor

Lock down omniauth-oauth2 version and a little refactor