[Android] Gradle: buildToolsVersion is not specified.

[Tokix]

It would be great if you could add the gradle version for the current plugin to your README.
In Gradle 3.3 compile was used and > Gradle 3.3 it is implementation.

Using Gradle 3.3 lead to the following Error in Cordova/Ionic:

Failed to notify ProjectEvaluationListener.afterEvaluate(), but primary configuration failure takes precedence. java.lang.IllegalStateException: buildToolsVersion is not specified.

with:

org.java-websocket:Java-WebSocket:1.3.9

If you should run into that issue too just change
implementation 'org.java-websocket:Java-WebSocket:1.3.9' to compile 'org.java-websocket:Java-WebSocket:1.3.9' or update your gradle version.