Add server_name param to boxen::project

Allows users to set the vhost name in the nginx config to something
other than $name.dev, but still defaults to the existing behavior.

manifests/project.pp

@@ -58,6 +58,7 @@
   $postgresql    = undef,
   $redis         = undef,
   $ruby          = undef,
+  $server_name   = "${name}.dev",
 ) {
   include boxen::config