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.
Implemented negative integers in NMatrix indexing; fixes #174 and
closes #181.
Merge commit '3a5eb34'
Implemented negative integers in NMatrix indexing; fixes #174 and
closes #181.
Merge branch 'rajatkapoor-negindex'

Conflicts:
	ext/nmatrix/ruby_nmatrix.c
Merge pull request #180 from hyaocuk/master

Minor fixes in Ruby version checking.
Ruby 2.1 support.
Merge branch 'master' of github.com:SciRuby/nmatrix