fix travis pr check

[techgaun]

based on the docs : https://docs.travis-ci.com/user/environment-variables/

TRAVIS_PULL_REQUEST: The pull request number if the current job is a pull request, “false” if it’s not a pull request.

This closes #199

[asmeurer]

I think the test failure is relevant here.

[techgaun]

oh crap I will check it out @asmeurer

[techgaun]

@asmeurer seems like this requires some more thought as I don't think

doctr/doctr/__main__.py

Line 270 in 421aefd

run(['git', 'checkout', get_travis_branch()])

would work. It would work for branch within the same repo but not on a different repo (like my fork for eg.). What do you suggest in this scenario?

[asmeurer]

Hmm. It seems really that should be use the merge commit generated by Travis, not the original branch.