[#8520] remove branches from .travis.yml

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

OpenProject members break their rules.
https://www.openproject.org/projects/openproject/wiki/Git_Workflow
> Fork Openproject on GitHub

DVCS standard rule is very simple.
https://help.github.com/articles/fork-a-repo

OpenProject stupid *undocumented* rule:
http://marutosi.bitbucket.org/RxTstudy-20130622/one-html/html/index.html#gitbarepush

DVCS standard rule:
http://marutosi.bitbucket.org/RxTstudy-20130622/one-html/html/index.html#id236

Git-flow defined only "master" and "develop" branches on *remote repository*.
http://nvie.com/posts/a-successful-git-branching-model/

And "release" and "hotfix" is local.
These local branches are *deleted* after pushing to remote repository.
OpenProject undocumented workflow is very *stupid*.

OpenProject undocumented stupid workflow takes much time.
Simple and standard DVCS workflow save much time.

There is no branches on Rails .travis.yml.
https://github.com/rails/rails/blob/2367dfeb8d7c4596263e/.travis.yml
Rails has many pull request.
https://github.com/rails/rails/pulls
Rails travis build is not bottleneck.
https://travis-ci.org/rails/rails/builds
So, OpenProject undocumented workflow is very stupid.

https://github.com/opf/openproject/pull/1362