gradle: Configure wrapper task to use .gradle-wrapper directory

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
gradle: Update gradlew to 2.1

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #15 from bjhargrave/gradlew-2.1

gradle: Update gradlew to 2.1