Skip threads tests unless perl version is 5.13.4 or greater

There was a long standing bug in the handling of GV <-> CV double linkage
that could (and explicitely did with a poisonous perl) cause segfaults at
thread destruction. It got fixed by Dave in commit 803f274 which went in
5.13.3, but the fix was amended for 5.13.4 in commit 09aad8f. Since it's
not really fair for the user to not be able to install the module because
of this, we skip the threads tests unless perl is at least 5.13.4.