Update vim-improvements.vundle

This removes a redundant Bundle that already appears on line 9. This was throwing warnings for me whenever I started macvim.
Merge pull request #470 from martco/patch-1

Update vim-improvements.vundle
Decided disabling arrow keys was hurting new people too much
Remove duplicated plugin
Merge branch 'master' into jby

* master:
  Remove duplicated plugin
  Decided disabling arrow keys was hurting new people too much
  Update vim-improvements.vundle