Version 1.3.1
* Updated the build to work with Android Studio 3.0 and Gradle 4.1.
* Target Android 6 (Marshmallow)
* When exporting or importing a file *and* on Marshmallow or higher,
  check whether the file location is outside of package-specific
  storage and if so request permission to access the folder if we
  don't already have it.
* When importing a file, also update any preferences that were saved.
* When the back button has been pressed in the note editor
  and the note has been changed, ask the user to confirm
  whether they want to discard the changes or cancel going back.
