Remove unused .travis.yml file

Summary: We are not running Travis, and have no plan to do so at the moment.

Test Plan:
Try to locate the .travis.yml in the source tree, and find yourself
failing this quest.

Reviewers: #bitcoin_abc, deadalnix, jasonbcox, markblundeberg

Reviewed By: #bitcoin_abc, jasonbcox, markblundeberg

Differential Revision: https://reviews.bitcoinabc.org/D3422