Update Travis CI to Android 26 sdk
Update Travis CI to Android 26 sdk

Nutomic/syncthing-android,jmintb/syncthing-android,syncthing/syncthing-android,syncthing/syncthing-android,jmintb/syncthing-android,flipreverse/syncthing-android,flipreverse/syncthing-android,Nutomic/syncthing-android,syncthing/syncthing-android