fix a bug in MatchData where it causes a crash due to an assertion when calls a method with uninitialized MatchData object

EX)
----
MatchData.allocate.regexp
[fix #174] use rb_vm_call instead of rb_funcall to avoid an issue #174 and improve a performance slightly
Merge branch 'master' of git://github.com/MacRuby/MacRuby into enable_gdbm