Merge pull request #72 from DirkR/master

renamed g:airline_fugitive_prefix to g:airline_branch_prefix in vimrc
Made vimrc compatible with Vim 7.2 on CentOS 6.4

The newest version of CentOS (6.4) ships with Vim 7.2.
Thus some 7.3 features break the vimrc on this "legacy"
platform.

I've put all broken statements (undo*, relativenumber, colorcolumn)
inside conditional statements.