bump version to 81

Change-Id: I18c389be334fea915acc2f23e89d2506b97be6b1
diff --git a/ring-android/app/src/main/AndroidManifest.xml b/ring-android/app/src/main/AndroidManifest.xml
index 9a26704..eb1c2f8 100644
--- a/ring-android/app/src/main/AndroidManifest.xml
+++ b/ring-android/app/src/main/AndroidManifest.xml
@@ -20,7 +20,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="cx.ring"
     android:installLocation="auto"
-    android:versionCode="80"
+    android:versionCode="81"
     android:versionName="20170531">
 
     <supports-screens