Use a gradle init script instead of mutating build.gradle files.
Merge pull request #17 from sourcegraph/immutable-build-dot-gradle

Use a gradle init script instead of mutating build.gradle files.
We depend on Java 8, since we use lambda expression.
Merge pull request #18 from sourcegraph/source-compatibility

We depend on Java 8, since we use lambda expression.
Added a test case for gradle projects.