Support gradle 4.1 and gradle android plugin 3.0

Due to an external lib we have to be very bleeding edge (nextcloud-dev) To still have a working (nearly nightly) build for our users it would be very cool if you can add support for

• gradle-4.1 which is already stable
• android gradle plugin 3.0, which currently is alpha, but as it is only if specifically asked for it should not bother other apps/builds

Both are available with the new repo "google()", which results in https://dl.google.com/dl/android/maven2/

Thank you for your great effort! Best regards, Tobias

P.S.: If I can help you with anything, please let me know.