Remove backup to prevent manifest merger conflicts

Not exactly sure if there was a reason for the "allowBackup" in the libraries manifest, but it seems to be the cause of a Gradle import issue. I don't know what version of Gradle this became an issue in, but it is present on my project using Gradle 0.11+ and Android Studio 0.6.1.

The error thrown is a manifest merge error. This change puts the manifest more in line with the [SackList manifest](https://github.com/commonsguy/cwac-sacklist/blob/master/sacklist/AndroidManifest.xml)

Currently, the solution to get around this is to set `useOldManifestMerger true` in your gradle build properties.

![screen shot 2014-06-14 at 7 33 39 pm](https://cloud.githubusercontent.com/assets/1014106/3279811/72e236ce-f41c-11e3-8850-e2307d585b25.png)