Update turtle pname checker to current turtle version

With the working draft 09 August 2011 (http://www.w3.org/TR/2011/WD-turtle-20110809/)
came a change in the turtle grammar specifically for turtle pnames (prefixed names)
as summarised in D.4 (http://www.w3.org/TR/turtle/#sec-changelog-Subm-WD)

- '.'s in names in all positions of a local name apart from the first or last, e.g. ex:first.name.
- digits in the first character of the PN_LOCAL lexical token, e.g. ex:7tm.

(raptor_turtle_is_legal_turtle_qname): adapt to current turtle standard