remove missed rvm syntax

dev/ci/tests/homebrew-packaging/setup

@@ -6,7 +6,7 @@ if ! gem list | grep -qE '^rack '; then
 	# which in turn requires the 'rack' gem to be installed in the system Ruby's
 	# default RubyGems path. 'rake test:install_deps' installs to a local bundler
 	# directory but not to the default RubyGems path.
-	retry_run 3 sudo system gem install rack --no-document
+	retry_run 3 sudo gem install rack --no-document
 fi
 retry_run 3 rake test:install_deps DEVDEPS_DEFAULT=no BASE_DEPS=yes SUDO=yes
 retry_run 3 gem install bundler -v '~>1.0' --no-document