Add billing permission for donations

astrid/src/main/AndroidManifest.xml

@@ -6,7 +6,7 @@
 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
     package="org.tasks"
     android:versionName="4.6.11"
-    android:versionCode="323">
+    android:versionCode="324">
 
     <!-- widgets, alarms, and services will break if Astrid is installed on SD card -->
     <!--    android:installLocation="internalOnly"> -->
@@ -39,6 +39,8 @@
     <uses-permission android:name="android.permission.RECORD_AUDIO" />
     <!-- Used to initiate sync when device comes back online -->
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <!-- required for in-app billing of donations -->
+    <uses-permission android:name="com.android.vending.BILLING" />
 
     <!-- ============================================== Exported Permissions = -->