bump Homebrew version to 0.9.8

So we can know whether a Homebrew installation is pre or post
core/formula separation.

Will update to 0.9.9 when moving core code to new repo.

Library/brew.sh

-HOMEBREW_VERSION="0.9.5"
+HOMEBREW_VERSION="0.9.8"
 
 odie() {
   if [[ -t 2 ]] # check whether stderr is a tty.