Upgraded Android Gradle Plugin and Gradle versions for Java 8-based build 

I tried to build this project under Java 8, and couldn't due to the error shown below.

This PR upgrades the Android Gradle Plugin and Gradle versions, which seems to help.

I have NOT yet run a full build through (currently download Android SDK through hotel WiFi..),
so please do double check that this doesn't break anything for you! ;)

Hope this helps,
Michael
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SMSSync$ ./gradlew clean

FAILURE: Build failed with an exception.

* Where:
Build file '/home/vorburger/dev/Saraswathi.TempAndNotInGit/SMSSync/sms-portals/build.gradle' line: 20

* What went wrong:
A problem occurred evaluating project ':sms-portals'.
> Ambiguous method overloading for method java.io.File#<init>.
  Cannot resolve which method to invoke for [null, class java.lang.String] due to overlapping prototypes between:
  	[class java.lang.String, class java.lang.String]
  	[class java.io.File, class java.lang.String]