FIX #41 "cwd" must be a string in Node 8

[zipang]

The fix is quite obvious, but i really really don't know what was happening before Node 8 decided to enforce the parameters validation of child_process.spawn() ?

[p-salido]

Tested on node 4:

octan% node -v
v4.8.6
octan% echo "console.log(require('process').cwd)" |node
[Function: cwd]
octan% echo "console.log(require('process').cwd())" |node
/home/w/apps/exp/firebase-server