[rubygem-narray/el6] fix gemspec on Fedora >= 21 remove `%%{gem_extdir_mri}/gem_make.out` again

[Björn Esser]

Summary of changes:

  2e4e635... fix gemspec on Fedora >= 21 remove `%%{gem_extdir_mri}/gem_ (*)

(*) This commit already existed in another branch; no separate mail sent