Simplify check for whether lxc is installed [GH-186]

lib/vagrant-lxc/provider.rb

@@ -26,14 +26,12 @@ module Vagrant
       end
 
       def ensure_lxc_installed!
-        unless system("which lxc-version > /dev/null 2>&1")
         begin
           sudo_wrapper.run("which", "lxc-version")
         rescue Vagrant::LXC::Errors::ExecuteError
           raise Errors::LxcNotInstalled
         end
       end
-      end
 
       # If the machine ID changed, then we need to rebuild our underlying
       # container.