Fixed setting of PID when starting Unicorn

From my experiences with my gem based on Guard::Unicorn, [Guard::Foreman] [gf], it seems that the block:

```ruby
@pid = Process.fork do
  system "#{cmd.join " "}"
end
```

Will create a child process to run the `system` command, which will, in fact, create yet another child process. In order to truly get the PID of the `Unicorn` process, I changed this block to:

```ruby
@pid = spawn "#{cmd.join " "}"
```

[gf]: https://github.com/J3RN/guard-foreman