Update contributor document to explain the transition to travis-ci.com

Travis CI is unifying their platform to be on travis-ci.com instead of .org - we should update our contributor document to reflect this change.

This was discussed during the Seattle network team hackfest in June 2018: https://trac.torproject.org/projects/tor/wiki/org/meetings/2018NetworkTeamHackfestSeattle/CI

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information