build on Travis-CI

[mfrasca]

File /home/travis/.android/repositories.cfg could not be loaded.

Checking the license for package Android SDK Build-Tools 26 in /usr/local/android-sdk/licenses

Warning: License for package Android SDK Build-Tools 26 not accepted.

FAILURE: Build failed with an exception.

* What went wrong:

A problem occurred configuring project ':app'.

> You have not accepted the license agreements of the following SDK components:

  [Android SDK Build-Tools 26].

  Before building your project, you need to accept the license agreements and complete
  the installation of the missing components using the Android Studio SDK Manager.

[mfrasca]

when done, activate the hook, it is now disabled.