Quote path in case $HOME has a space in it.

I've seen stranger things on OS X, unfortunately.
This reverts commit 1f5cecee4768be192e439a72a873a6cfe8720927.

lib/nvm.zsh

 # get the node.js version
 function nvm_prompt_info() {
-  [ -f $HOME/.nvm/nvm.sh ] || return
+  [ -f "$HOME/.nvm/nvm.sh" ] || return
   local nvm_prompt
   nvm_prompt=$(node -v 2>/dev/null)
   [[ "${nvm_prompt}x" == "x" ]] && return