Clean up ivy and sbt artifacts
Clean up ivy and sbt artifacts

Fixes https://github.com/travis-ci/travis-ci/issues/5016

These files exist on the base JVM build image, so even when they are
removed in `before_cache`, we would detect changes and end up
uploading a new cache every time.

JuliaCI/travis-build,craigcitro/travis-build,akoeplinger/travis-build,JuliaCI/travis-build,jhass/travis-build,jhass/travis-build,akoeplinger/travis-build,andyli/travis-build,kevmoo/travis-build,craigcitro/travis-build,kevmoo/travis-build,andyli/travis-build