support diff id as param - fix revision id parsing when null revision

[abhishekparwal]

when passed a preview diff which has a diff ID but no revision ID. it is failing with ClassClassexception on trying to cast it as String.

java.lang.ClassCastException: net.sf.json.JSONNull cannot be cast to java.lang.String
	at com.uber.jenkins.phabricator.conduit.Differential.getRevisionID(Differential.java:58)
	at com.uber.jenkins.phabricator.conduit.Differential.decorate(Differential.java:81)
	at com.uber.jenkins.phabricator.PhabricatorNotifier.perform(PhabricatorNotifier.java:240)
	at hudson.tasks.BuildStepCompatibilityLayer.perform(BuildStepCompatibilityLayer.java:81)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:744)
	at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:690)
	at hudson.model.Build$BuildExecution.post2(Build.java:186)
	at hudson.model.AbstractBuild$AbstractBuildExecution.post(AbstractBuild.java:635)
	at hudson.model.Run.execute(Run.java:1841)
	at com.tikal.jenkins.plugins.multijob.MultiJobBuild.run(MultiJobBuild.java:76)
	at hudson.model.ResourceController.execute(ResourceController.java:97)
	at hudson.model.Executor.run(Executor.java:429)

[coveralls]

Coverage decreased (-0.03%) to 89.949% when pulling 8638ac0 on fix_revision_id_parse into 80b996f on master.