Added missing build-helper-maven-plugin version.

Fixes the following maven warning:

```
[WARNING]
[WARNING] Some problems were encountered while building the effective model for com.octo.android:android-sample-robolectric-tests:apk:0.0.1-SNAPSHOT
[WARNING] 'build.plugins.plugin.version' for org.codehaus.mojo:build-helper-maven-plugin is missing. @ line 114, column 12
[WARNING]
[WARNING] It is highly recommended to fix these problems because they threaten the stability of your build.
[WARNING]
[WARNING] For this reason, future Maven versions might no longer support building such malformed projects.
[WARNING]
```