Update JobInvocation.groovy

Version 0.14 of the plugin has a bug where trying to run a job (at least with parameters) causes an exception that property "displayName" is missing. 

This property does not seem to exist on JobInvocation, I am guessing it used to.   

I worked around this issue in my Jenkins script by using metaprogramming, but here is a PR that should make the problem go away.