Made vimrc gracefully degrade for servers with older vim versions.

Added comment to rtp line.
Added `if exists` test for relativenumber before setting it.
Merge branch 'master' of github.com:zanshin/dotfiles

* 'master' of github.com:zanshin/dotfiles:
  Refactored bashrc for multiple operating systems.
  Changed push.default from `simple` to `matching`.