Cache downloaded external dependencies and Gradle Wrapper (#1202)
Cache downloaded external dependencies and Gradle Wrapper (#1202)

As suggested by Travis CI, cache downloaded, external dependencies
and Gradle Wrapper. Subsequent builds will reuse existing files which
should significantly speed up CI execution.

https://docs.travis-ci.com/user/languages/java/#Caching
sbrannen/junit-lambda,junit-team/junit-lambda