bootstrap.sh: cd back to previous directory after complete

`cd $OLDPWD` prevents echoing the directory we are cd'ing to.

This is similar to the first commit from https://github.com/mathiasbynens/dotfiles/pull/26, and allows you to conveniently run the bootstrap from anywhere and continue what you were doing. I didn't experience any side effects running bootstrap from my dotfiles repo, and this works with and without `source`.