Update Android Plugin for Gradle to version 2.1.0.

Upgrade of gradle to 2.13 was causing some issues.

build.gradle

@@ -3,7 +3,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.1.0'
+        classpath 'com.android.tools.build:gradle:2.1.0'
         classpath 'com.jfrog.bintray.gradle:gradle-bintray-plugin:1.5'
     }
 }