Use exec when running to a new process.

Basically, have /usr/bin/vagrant exec /opt/vagrant/bin/vagrant which
should then exec ruby. Keeps everything in one process instead of 3
separate ones.

This helps avoid this process listing:
  15964 bash /usr/bin/vagrant up
  15965 bash /opt/vagrant/bin/vagrant up
  15971 /opt/vagrant/bin/../embedded/bin/ruby /opt/vag...<trimmed>