ruby: Install certificate for +openssl only (#11184)

If specifying ~openssl, the post-install step will still try to install
the non-existent certificate, leading to an IOError.

var/spack/repos/builtin/packages/ruby/package.py

@@ -96,6 +96,7 @@ def post_install(self):
         http://guides.rubygems.org/ssl-certificate-update/
         for details.
         """
+        if self.spec.satisfies("+openssl"):
             rubygems_updated_cert_path = join_path(self.stage.source_path,
                                                    'rubygems-updated-ssl-cert',
                                                    'GlobalSignRootCA.pem')