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.
Merge branch 'master' of github.com:zaiste/vimified

* 'master' of github.com:zaiste/vimified:
  Update README.
  Add Elixir support.
Merge branch 'master' of git://github.com/zaiste/vimified

* 'master' of git://github.com/zaiste/vimified:
  Update README.
  Add Elixir support.
  renamed g:airline_fugitive_prefix to g:airline_branch_prefix in vimrc
Merge branch 'master' of github.com:DirkR/vimified

* 'master' of github.com:DirkR/vimified:
  Made vimrc compatible with Vim 7.2 on CentOS 6.4
Merge pull request #75 from DirkR/master

Made vimrc compatible with Vim 7.2 on CentOS 6.4