Fixes assignment to const RHASH_IFNONE

This addresses the following issue:
https://github.com/SciRuby/nmatrix/issues/176

In Ruby 2.1.0, RHASH_IFNONE is a const, and instead of
directly setting it, you call RHASH_SET_IFNONE.
Merge pull request #178 from diminish7/RHASH_IFNONE_fix

Fixes assignment to const RHASH_IFNONE; resolves Ruby 2.1+ compilation error and preserves backwards compatibility.