Update to RetroFit 1.7.1, fix compilation issues with 500px example

In #65, we moved from setServer() to setEndpoint(). However, that method does not exist in the Retrofit 1.3.0 we were using. Update to Retrofit 1.7.1 to gain access to the setEndpoint() method. Also fixes the deprecation of isNetworkError().
Merge pull request #92 from ianhanniballake/update_500px_retrofit

Update to RetroFit 1.7.1, fix compilation issues with 500px example