Fix auto upgrade failure from non-exported ZSH env var

Fixes #549. Specify ZSH=$ZSH explicitly when invoking
the auto update scripts.

oh-my-zsh.sh

 # Check for updates on initial load...
 if [ "$DISABLE_AUTO_UPDATE" != "true" ]
 then
-  /usr/bin/env zsh $ZSH/tools/check_for_upgrade.sh
+  /usr/bin/env ZSH=$ZSH zsh $ZSH/tools/check_for_upgrade.sh
 fi
 
 # Initializes Oh My Zsh

tools/check_for_upgrade.sh

@@ -24,7 +24,7 @@ then
     read line
     if [ "$line" = Y ] || [ "$line" = y ]
     then
-      /bin/sh $ZSH/tools/upgrade.sh
+      /usr/bin/env ZSH=$ZSH /bin/sh $ZSH/tools/upgrade.sh
       # update the zsh file
       _update_zsh_update
     fi