Add maven plugin to fix issue with missing install task

jitpack.io complains about the missing task 'install' and
suggests the installation of the maven plugin.

Author: eberhardtj

build.gradle

@@ -3,6 +3,7 @@ version '0.4.0'
 
 apply plugin: 'java'
 apply plugin: 'java-library'
+apply plugin: 'maven'
 
 sourceCompatibility = 1.8
 targetCompatibility = 1.8