Upgrade gradle to benefit from instant run 'again'.

Now it needs to be 2.1.2 to support instant run.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125445366

build.gradle

@@ -20,7 +20,7 @@ buildscript {
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.1.0'
+        classpath 'com.android.tools.build:gradle:2.1.2'
         classpath 'com.novoda:bintray-release:0.3.4'
     }
 }