Add support for conv.followup() in dashbot-google

[andyvanhelsing]

When conv.followup() is used in the actions-on-google dialogflow library the following error occurs:

"validation error: {"success":false,"errors":[{"message":"at least one of [items[0].simpleResponse.displayText or speech or expectedInputs[0].inputPrompt.initialPrompts[0].ssml or expectedInputs[0].inputPrompt.initialPrompts[0].textToSpeech or richResponse.items[0].simpleResponse.displayText or richResponse.items[0].simpleResponse.ssml or expectedInputs[0].inputPrompt.richInitialPrompt.items[0].simpleResponse.textToSpeech or finalResponse.speechResponse.textToSpeech or fulfillmentText or data.google.richResponse.items[0].simpleResponse.displayText or data.google.richResponse.items[0].simpleResponse.ssml or data.google.richResponse.items[0].simpleResponse.textToSpeech or payload.google.richResponse.items[0].simpleResponse.textToSpeech or payload.google.richResponse.items[0].simpleResponse.displayText or payload.google.richResponse.items[1].simpleResponse.textToSpeech or payload.google.richResponse.items[1].simpleResponse.displayText or fulfillmentMessages[0].simpleResponses.simpleResponses[0].textToSpeech or fulfillmentMessages[0].simpleResponses.simpleResponses[0].displayText] needs to be present in post body"}]}"

I can hide this error using the printErrors = false configuration but it would be best if followup events were handled natively