Fix sourcing of asdf.sh (#710)

Installing asdf via Homebrew now requires souring from a different path.

This same issue was recently fixed in thoughbot/laptop by @cpytel, and so I've copied the changes from this commit:
aa3a84e15e

zsh/configs/post/path.zsh

@@ -5,9 +5,7 @@ PATH="$HOME/.bin:/usr/local/sbin:$PATH"
 if [ -f "$HOME/.asdf/asdf.sh" ]; then
   . "$HOME/.asdf/asdf.sh"
 elif which brew >/dev/null &&
-  BREW_DIR="$(dirname `which brew`)/.." &&
-  [ -f "$BREW_DIR/opt/asdf/asdf.sh" ]; then
-  . "$BREW_DIR/opt/asdf/asdf.sh"
+  . "$(brew --prefix asdf)/libexec/asdf.sh"
 fi
 
 # mkdir .git/safe in the root of repositories you trust