upgrade to Gradle 1.7

When using version 1.1.0 under Gradle 1.7, the following warnings appear:
```
The ConfigurationContainer.add() method has been deprecated and is scheduled to be removed in Gradle 2.0. Please use the create() method instead.
The TaskContainer.add() method has been deprecated and is scheduled to be removed in Gradle 2.0. Please use the create() method instead.
```

This pull request upgrades the project itself to Gradle 1.7, and fixes the deprecation warnings.

cc: @mbruner