remove dev-angular branch from .travis.yml

Git-flow recommends to use "--no-ff".
http://nvie.com/posts/a-successful-git-branching-model/

But dev-angular reffers to same revision with dev.

<pre>
$ git branch -v -r --abbrev=9 | grep gh-finn/dev
  gh-finn/dev           cc5bedac9 Merge pull request #1386 from opf/bugfix/icon-font-toggler
  gh-finn/dev-angular   cc5bedac9 Merge pull request #1386 from opf/bugfix/icon-font-toggler
</pre>