0.3.0 doesn't work on Spark 2.3 due to protobuf version conflict

[mengxr]

Spark depends on protobuf-java 2.5.0 but corenlp depends on 3.2.0. Error:

java.lang.VerifyError: Bad type on operand stack
Exception Details:
  Location:
    com/google/protobuf/GeneratedMessageV3$ExtendableMessage.getExtension(Lcom/google/protobuf/GeneratedMessage$GeneratedExtension;I)Ljava/lang/Object; @3: invokevirtual
  Reason:
    Type 'com/google/protobuf/GeneratedMessage$GeneratedExtension' (current frame, stack[1]) is not assignable to 'com/google/protobuf/ExtensionLite'
  Current Frame:
    bci: @3
    flags: { }
    locals: { 'com/google/protobuf/GeneratedMessageV3$ExtendableMessage', 'com/google/protobuf/GeneratedMessage$GeneratedExtension', integer }
    stack: { 'com/google/protobuf/GeneratedMessageV3$ExtendableMessage', 'com/google/protobuf/GeneratedMessage$GeneratedExtension', integer }
  Bytecode:
    0x0000000: 2a2b 1cb6 0024 b0

We need to try shading spark-corenlp dependencies, which might take some work.