parallelshell issue #22

Failing case for parallelshell.

$ git clone https://github.com/nichoth/para-test.git
$ cd module-1
$ npm install
$ npm run dev

In a separate terminal:

$ cd module-2
$ npm install
$ npm run dev

node server.js throws error because the first server is using the same port, but watchify still runs.