linux & mac: update RubyGems before updating gems
.gitconfig: make `git rebase` safer on a Mac

from mathiasbynens/dotfiles@b36561c2df2b9f92f237fdfdd0e1697c7239826e
Merge branch 'master' of git://github.com/phette23/my-dotfiles

* 'master' of git://github.com/phette23/my-dotfiles:
  readme: don't need to escape underlines in bash_profile