Use recommended gradle build configuration

The build.gradle in use was auto-generated by Android Studio and
differs slightly from the official documentation[1]. In an effort to
improve sanity on this project, this patch brings build.gradle as
close as possible to the recommended one.

The only differences between our build.gradle and the one in the
official documentation are:

 * android.tools.build:gradle:0.5.x to compile with gradle above 1.6.
 * lib/android-support-v4.jar is not used. The dependency is instead
   fetched from the internet by gradle.
 * Use mavenCentral() as a repository instead of hardcoding its
   address. This is a standard gradle practice[2].

[1] https://developers.facebook.com/docs/getting-started/facebook-sdk-for-android-using-android-studio/3.0/
[2] http://www.gradle.org/docs/current/userguide/dependency_management.html#sub:maven_central