Vagrant 1.3 compatibility #3

using the current Vagrantfile with vagrant 1.3.4 gives the following error:

• The 'extra' flag on synced folders is now 'mount_options'

changing extra into mount_options throws another error, so we should check for general compatibility or state that only a specific vagrant version is supported.

More details about 1.3:
http://vagrant.1086180.n5.nabble.com/ANN-Vagrant-1-3-0-Released-td1627.html