fixes gradle error while building apk

[EmmanuelAB]

This fixes the error:
Error:Failed to complete Gradle execution. Cause: The version of Gradle you are using (2.14.1) does not support the forTasks() method on BuildActionExecuter. Support for this is available in Gradle 3.5 and all later versions.
When trying to build the APK from Android Studio

[nguillaumin]

Fair enough, although I'm not sure you're supposed to edit the Gradle Wrapper properties file like so? (I'm not very familiar with it).

The Travis build fail though, but I'm not sure the error is directly related or if it's just a transient one.

[jamescr]

The error that @EmmanuelAB comment is happening when generation the APK file on Android Studio version 3.0+. The Android Studio upgrade requires a gradle update.

Fair enough, although I'm not sure you're supposed to edit the Gradle Wrapper properties file like so? (I'm not very familiar with it).

I'm also not sure completely about editing the gradle properties file.

The Travis build fail though, but I'm not sure the error is directly related or if it's just a transient one.

An issue related with this was reported to Travis last year and it stills open.

And it seems something is happening with Travis because the Pull Request #130, that only modifies the readme file, has also failed.