Add Gradle wrapper to allow building by Jitpack

[jahunt1]

This change adds the Gradle wrapper to the project. Having the Gradle wrapper under VCS will allow the latest versions of the SDK to be built by Jitpack.io. I think this is desirable since jcenter() is no longer publicly available.

The latest builds for this repository have failed: https://jitpack.io/#kissmetrics/KISSmetrics-Android-SDK.

Looking at the log for the 2.3.1 build:

Found gradle
Gradle build script
WARNING: gradle/wrapper/gradle-wrapper.jar does not exist! Needs to be committed.

ERROR: Gradle wrapper not found. Please add. Using default gradle to build.

When building the script falls back to Gradle 4.8.1 which is not recent enough to be able to build the jars.