use the new .has_plugin method of the Vagrant object to optionally enable plugins

The .has_plugin method of the Vagrant object is available if your installed version of Vagrant is 1.3.2 or higher, to optionally enable the Landrush and Vagrant Cachier plugins, if they are installed. This PR will therefore make Vagrant 1.3.2 or higher a requirement in order to utilize the vagrant-dspace project.