use `rb_funcall2` instead of `rb_funcallv`

In 2.1 `rb_funcall2`  was renamed to `rb_funcallv` and
a macro for backward compatibility was added.
Merge pull request #32 from cremno/use-rb_funcall2-instead-of-rb_funcallv

use `rb_funcall2` instead of `rb_funcallv`