Add missing definition for Sass::Script::Tree::Selector

This was causing a crash when someone had code like the following:

    .class#{&} {
      ...
    }

Fix by adding to the list of classes for which `visit_method` must be
defined.

Change-Id: Ia914c24da74d14cb4d12ccba03d1f4efc606e85f
Reviewed-on: http://gerrit.causes.com/42572
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>