This is a sign-post issue. jruby-head does not currently build right. The setup-ruby step say this: Modifying JAVA_HOME for JRuby attempting to run with existing JAVA ...

ERROR:java.lang.UnsupportedClassVersionError: burp/BurpExtender has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtime only ...