Re-enable system testing in travis matrix

[dgrove-oss]

The core openwhisk upgrade to akka 2.6.12 included changes to how the test system handled https that broke our ability to externally run the openwhisk test suite. The fix appears to be non-trivial, so https://github.com/apache/openwhisk-deploy-kube/pull/700/commits/94ea0610c514dccfba3fd467f9da0ac1428f319d disabled system testing in this repo to unblock merging PRs. We do need to eventually figure out a way to make this work and restore the testing.