Update to gradle-plugin 2.2.1

Test: ./gradlew assemble assembleAndroidTest
Change-Id: I2d97beb1d362a138920d1c242d562b5f55ede50a
diff --git a/build.gradle b/build.gradle
index 1442e38..dbcf59f 100644
--- a/build.gradle
+++ b/build.gradle
@@ -15,7 +15,7 @@
     }
     dependencies {
         // Keep gradle plugin version in sync with ub_supportlib-master manifest.
-        classpath 'com.android.tools.build:gradle:2.2.0'
+        classpath 'com.android.tools.build:gradle:2.2.1'
     }
 }