Updated gradle + plugins

Updated Gradle, Gradle plugin and Bintray plugin to newest versions.
Added realm-annotation-processor project to settings.gradle, so it appears as a editable project inside Android Studio.

Note: You might have to delete the .gradle folder inside the project after updating to avoid the following error:

````
org.gradle.api.internal.changedetection.state.DefaultFileCollectionSnapshotter$FileCollectionSnapshotImpl cannot be cast to org.gradle.api.internal.changedetection.state.OutputFilesCollectionSnapshotter$OutputFilesSnapshot
````

We'll need to update this file also for the realm-jni and realm-annotations-processor projects