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
Update vimrc

I think the NERDTree window should not resize between multiple toggles.  Makes it difficult to read the longer file names.
Check for git and vim before running the install.sh script.
Merge pull request #76 from xiaogwu/master

Update vimrc
Merge pull request #77 from DirkR/installer_improvements

Check for git and vim before running the install.sh script.
Use default colorscheme during installation.
Merge pull request #78 from DirkR/installer_improvements

Use default colorscheme during installation.