Fixed groovy.lang.MissingFieldException: No such field: uid

Fixes the following error:
        ...
	at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1061)
	at hudson.util.XStream2.unmarshal(XStream2.java:109)
	at com.thoughtworks.xstream.XStream.unmarshal(XStream.java:1045)
	at hudson.XmlFile.unmarshal(XmlFile.java:166)
	... 144 more
Caused by: groovy.lang.MissingFieldException: No such field: uid for class: com.cloudbees.plugins.flow.JobInvocation
	at groovy.lang.MetaClassImpl.getAttribute(MetaClassImpl.java:2482)
	at groovy.lang.MetaClassImpl.getAttribute(MetaClassImpl.java:3308)
	at org.codehaus.groovy.runtime.InvokerHelper.getAttribute(InvokerHelper.java:127)
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.getField(ScriptBytecodeAdapter.java:302)
	at com.cloudbees.plugins.flow.JobInvocation$Start.propertyMissing(JobInvocation.groovy)
	at sun.reflect.GeneratedMethodAccessor79.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at org.codehaus.groovy.reflection.CachedMethod.invoke(CachedMethod.java:90)
	at groovy.lang.MetaClassImpl.invokeMissingProperty(MetaClassImpl.java:752)
	at groovy.lang.MetaClassImpl$11.getProperty(MetaClassImpl.java:1761)
	at org.codehaus.groovy.runtime.callsite.GetEffectivePogoPropertySite.callGroovyObjectGetProperty(GetEffectivePogoPropertySite.java:67)
	at com.cloudbees.plugins.flow.JobInvocation.equals(JobInvocation.groovy:236)
	at java.util.HashMap.put(HashMap.java:376)
	at com.thoughtworks.xstream.converters.collections.MapConverter.putCurrentEntryIntoMap(MapConverter.java:93)
	at com.thoughtworks.xstream.converters.collections.MapConverter.populateMap(MapConverter.java:78)
	at com.thoughtworks.xstream.converters.collections.MapConverter.populateMap(MapConverter.java:72)
	at com.thoughtworks.xstream.converters.collections.MapConverter.unmarshal(MapConverter.java:67)
	at com.thoughtworks.xstream.core.TreeUnmarshaller.convert(TreeUnmarshaller.java:72)
	... 171 more