Modify Vagrantfile to gracefully allow sans-plugin

Previously, running a command such as `vagrant plugin list` while in a directory with an Oscar-template-generated Vagrantfile would cause Vagrant to error out, as the Vagrant.require_plugin('oscar') command would omit the plugin due to being run in a non-plugin-enabled mode, and the subsequent reference to the Oscar class would then error out.

This commit appends `if defined? Oscar` to the `Vagrant.configure` call, thus gracefully no-op'ing if plugin loading is disabled.