The Travis post build is only for the main repo

Skip it if it's PR testing or not the main repo
Correct PR testing env variable

Missed that it's not true if it's a PR from the docs
'TRAVIS_PULL_REQUEST: The pull request number if the current job is a
pull request, "false" if it's not a pull request.'
Merge pull request #1024 from nschonni/skip-travis-fetch-not-main-repo

The Travis post build is only for the main repo