fix vagrant vm forward_port

fix vagrant vm forward_port:
There is a syntax error in the following Vagrantfile. The syntax error
message is reproduced below for convenience:

/opt/vagrant-phalcon/Vagrantfile:6: syntax error, unexpected ':', expecting kEND
  config.vm.network :forwarded_port, guest: 80, host: 8081
                                           ^

compared with: http://framework.zend.com/svn/framework/standard/trunk/Vagrantfile