gradle: update tools build version

Change-Id: I78a941966b3a1978766f7a113638b1748806c84a
diff --git a/ring-android/build.gradle b/ring-android/build.gradle
index fc5195d..591953f 100644
--- a/ring-android/build.gradle
+++ b/ring-android/build.gradle
@@ -4,7 +4,7 @@
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.2.1'
+        classpath 'com.android.tools.build:gradle:2.2.2'
         classpath 'com.neenbedankt.gradle.plugins:android-apt:1.8'
 
         // NOTE: Do not place your application dependencies here; they belong