Migrate Travis CI to travis-ci.com

[lipnitsk]

travis-ci.org has the following notice:

Please be aware travis-ci.org will be shutting down in several weeks, with all accounts migrating to travis-ci.com. Please stay tuned here for more information.

https://docs.travis-ci.com/user/migrate/open-source-repository-migration

While travis-ci.org appears to still run builds they seem to take much longer than before.