Use awk instead of sed to filter ever Nth line out of the integration test output The sed -n "K~N" syntax is not compatible with sed on MacOS.

Testing Done:
compared the list of integration tests listed when modifying the -i N:M parameters to the script.
Here is a good CI run https://travis-ci.org/pantsbuild/pants/builds/41359083

Bugs closed: 800

Reviewed at https://rbcommons.com/s/twitter/r/1350/