Vim airline branch not showing

Your .vimrc file looks fine. The problem is probably with plugins implementation, because there were some recent changes in airline initialization. Immediate solution to your problem would be to move back a little bit in git history of an airline plugin, as far as I know just the last commit causes problem, so you can just type in:

$ cd ~/.vim/bundle/vim-airline
$ git checkout HEAD~1

If you want to stay up to date with the newest changes, monitor them on github, and run :PluginUpdate (for Vundle installation) or just git pull origin master to get newest features. If problem still exists, just go back a few commits or omit the one causing problems.

I hope it helps :)

My reputation isn't high enough to add a comment to quibix's solution hence this comment. The following commit is the last working airline commit that supports the fugitive status line addition. It does appear to break other plugins.

$git checkout e2a120869ba36da5d26df74fb23ef4052d55b6f0

EDIT

Actually fugitive#head() is on the master vim-fugitive branch. I figured out my problem was that fugitive was loaded after airline so airline_section_b was set to ''. Once I renamed the directory so that fugitive loaded before airline, HUNK/BRANCH started loading in the status line.