[#8406] remove branches from .travis.yml

https://www.openproject.org/work_packages/8406

It is too hard to run travis on forked repositories.
It disturbs pull request process.

Travis does not run Pull Request for Pull Request.
https://github.com/opf/openproject/pull/1392

For keeping clean revision history.
https://github.com/opf/openproject/pull/1338#issuecomment-44260428