Improve CI caching.

Add an option to skip pants bootstrap pre-clean
allowing cached build-support/pants_dev_deps.venv
to survive and be utilized.

I missed this cleaning going on in ci.sh when 1st enabling caching in https://rbcommons.com/s/twitter/r/1537/
This change actually uses the build-support/pants_dev_deps.venv now to avoid pip installing pants deps unless they change as evidenced by logs.

Testing Done:
CI went green here: https://travis-ci.org/pantsbuild/pants/builds/44795443

Bugs closed: 888

Reviewed at https://rbcommons.com/s/twitter/r/1538/
Sync of https://github.com/pantsbuild/pants/commit/e08dc7ebf369c5dcabc532ae872d75e809b62096