Cirrus: Use sudo to install Ruby gems

See cirruslabs/macos-image-templates#19

Author: sanssecours

.cirrus.yml

@@ -152,7 +152,7 @@ mac_task:
       brew install [email protected]
       export PATH="/usr/local/opt/[email protected]/bin:/usr/local/lib/ruby/gems/2.6.0/bin:$PATH"
     - | # Install Ruby gems
-      gem install ronn test-unit --user-install --no-document
+      sudo gem install ronn test-unit --no-document
 
   script:
     - > # Use latest version of LLVM to translate Elektra