sort out Travis-CI situation

[SethTisue]

in this repo in particular, the move to travis-ci.com didn't go smoothly — we ended up with both .org and .com enabled, and the publishing secrets didn't carry over, so I'll have to redo that

and I'll want to review the branch protection rules, I set one up quickly just now, but I'll want to go over it carefully

the first PR that was affected by this was #17

[SethTisue]

I think this is good now. we can always tweak the branch protection stuff further, as needed