Fix function name in fish_prompt

[e-nomem]

One of the references to the old function name _old_fish_prompt was left behind, and this completely breaks the prompt in fish.

It looks like this:

eashwar@laptop ~/s/homepage (master)> . nodeenv/bin/activate.fish
fish: Unknown command: _old_fish_prompt
nodevenv/bin/activate.fish (line 131): 
        _old_fish_prompt
        ^
in function 'fish_prompt'
in command substitution

It would be nice if a patch release could be made after merging this fix, because manually fixing each nodeenv after creation is quite annoying.