Explicitly set express's server port with grunt

My local environment is littered with lots of running servers
(I know, it's a bad habit for security! I'll try to be better.)
Anyway, in order to start this project, I tried to set env.PORT
to a freely available port on my machine before running
`grunt serve`. It worked:

```bash
PORT=12345 grunt serve
```

Unfortunately, it didn't work with `grunt dev`, presumably
because background processes don't env variables from their parents
in the same way. Anyway, this pull request should make the following
work too:

```bash
PORT=12345 grunt dev
```