Fix rvm_wrapper transient variables

The transient? method is passed a string, not a symbol, as far as I can tell.

This appears to fix JENKINS-18841 for me so far (the bug is sometimes hard to reproduce).

See:
https://github.com/jenkinsci/jruby-xstream/blob/master/src/main/java/org/jenkinsci/jruby/JRubyXStreamConverter.java#L61