IBrowserDefault fix

on older version of plone (in this case Plone 4.1) the comment content type declares it implementes IBrowserDefault, however, getLayout() is not implemented and returns a bad value (from the parent) which causes an attribute error such as the folloing from TinyMCE 1.3.5

        template = None
        if IBrowserDefault.providedBy(context):
            template = context.unrestrictedTraverse(context.getLayout())

This results in a attribute error in the unrestrictedTraverse:
    *** AttributeError: conversation_browserview

this update returns a sain value for getLayout() in newer versions of Plone the comment doesn't seem to inherit this IBrowserDefault interface so this is more of a backwards compatibility fix.