Increase forkCountTestPackage for sudo-enabled TravisCI

The switch from the container-based to sudo-enabled environment in TravisCI has increased available memory from 4 GB to 7.5 GB so use a forkCount of 2 in all packages including flink-test. See https://docs.travis-ci.com/user/ci-environment/

The sudo-enabled machines look to be Google Compute Engine n1-standard-2 with 2 "virtual CPUs". See https://cloud.google.com/compute/pricing.