Change default zshrc to export $ZSH (as required by check_for_upgrade.sh). 

I find that tools/check_for_upgrade.sh can’t find tools/upgrade.sh unless I export $ZSH in my zshrc. This commit just changes the zshrc template to export $ZSH. 

I’m using an up-to-date version of the master branch of oh-my-zsh on zsh 4.3.9 (i386-apple-darwin10.0). The error text I see when I don’t export $ZSH is: 

[Oh My Zsh] Would you like to check for updates?
Type Y to update oh-my-zsh: y
/bin/sh: /tools/upgrade.sh: No such file or directory