On develop.start, kill child process if one is already alive

This enables use of a `watch` task for monitoring changes to any server files, and restarting the server whenever those changes occur. In order to do so, the previous child process must be killed. Furthermore, the `watch` task must be called with the `nospawn` option set, as per:

    watch: {
      develop: {
        files: ['<%= jshint.server %>'],
        tasks: 'develop',
        options: {
          nospawn: true
        }
      }
    }