(CFACT-158) Use rb_block_call instead of rb_funcall_with_block.

rb_funcall_with_block was added in a later 1.9.3 patch.  To support all
1.9.3 versions, use rb_block_call instead.

rb_block_call provides the exact same functionality we need, but doesn't
require us to wrap the C func in a Ruby proc.
Merge pull request #221 from peterhuene/cfact/158

(CFACT-158) Use rb_block_call instead of rb_funcall_with_block.