Update README.md to reflect Vagrant version needed
Update README.md to reflect Vagrant version needed

Since v0.4.0 release this plugin will only work with Vagrant 1.5 or
higher. Update the documentation to reflect this.

nicolasbrechet/vagrant-cloudstack,MissionCriticalCloud/vagrant-cloudstack,atsaki/vagrant-cloudstack,j00p34/vagrant-cloudstack,schubergphilis/vagrant-cloudstack,MissionCriticalCloud/vagrant-cloudstack,miguelaferreira/vagrant-cloudstack,bheuvel/vagrant-cloudstack,schubergphilis/vagrant-cloudstack,nicolasbrechet/vagrant-cloudstack,j00p34/vagrant-cloudstack,miguelaferreira/vagrant-cloudstack,bheuvel/vagrant-cloudstack,atsaki/vagrant-cloudstack