Fix auto swtich to ruby-mode on .rake, .gemspec and Gemfile

Looks like this got fixed in master before and then the changes got lost. Just a hunch, but I suspect when @samaaron merged @balinterdi's initial PR, he didn't realize the changes were only in the live pack directory and not the dev pack as well, thus clobbering the fix when the update-live-packs script was run.

This PR applies the same change as @balinterdi's original commit to both the dev and the live packs.

Cheers!