User reports protobuf ClassChangeError running against 2.6.0 or above

https://stackoverflow.com/questions/55146663/issue-while-running-dataflow/55171101?noredirect=1#comment97081527_55171101

```

java.lang.IncompatibleClassChangeError: Class org.apache.beam.model.pipeline.v1.RunnerApi$StandardPTransforms$Primitives
does not implement the requested interface com.google.protobuf.ProtocolMessageEnum at org.apache.beam.runners.core.construction.BeamUrns.getUrn(BeamUrns.java:27)
at org.apache.beam.runners.core.construction.PTransformTranslation.(PTransformTranslation.java:58) at
org.apache.beam.runners.core.construction.UnconsumedReads$1.visitValue(UnconsumedReads.java:49) at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit(TransformHierarchy.java:666)
at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit(TransformHierarchy.java:649) at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit(TransformHierarchy.java:649)
at org.apache.beam.sdk.runners.TransformHierarchy$Node.visit(TransformHierarchy.java:649) at org.apache.beam.sdk.runners.TransformHierarchy$Node.access$600(TransformHierarchy.java:311)
at org.apache.beam.sdk.runners.TransformHierarchy.visit(TransformHierarchy.java:245) at org.apache.beam.sdk.Pipeline.traverseTopologically(Pipeline.java:458)
at org.apache.beam.runners.core.construction.UnconsumedReads.ensureAllReadsConsumed(UnconsumedReads.java:40)
at org.apache.beam.runners.dataflow.DataflowRunner.replaceTransforms(DataflowRunner.java:868) at org.apache.beam.runners.dataflow.DataflowRunner.run(DataflowRunner.java:660)
at org.apache.beam.runners.dataflow.DataflowRunner.run(DataflowRunner.java:173) at org.apache.beam.sdk.Pipeline.run(Pipeline.java:313)
at org.apache.beam.sdk.Pipeline.run(Pipeline.java:299)

```

Imported from Jira [BEAM-6839](https://issues.apache.org/jira/browse/BEAM-6839). Original Jira may contain additional context.
Reported by: kenn.