Update gradle android tool to 3.1.3

Thanks to Monte48 for the correct maven URL outside of android studio

build/android/build.gradle

@@ -3,9 +3,10 @@ buildscript {
 		mavenCentral()
 		jcenter()
 		google()
+		maven { url 'https://maven.google.com' }
 	}
 	dependencies {
-		classpath "com.android.tools.build:gradle:2.3.0"
+		classpath "com.android.tools.build:gradle:3.1.3"
 	}
 }