Add Instagram Support

[gus-bn]

Instagram Support

• Adds Support for Instagram by adding a new channel: Messaging::Channels::Instagram
• Tests for Messages contents edge cases for text, image, audio and video

Usage

vonage_client = Vonage::Client.new(...) # auth credentials
message = Vonage::Messaging::Message.instagram(
  type: 'text',
  message: 'This is an Instagram text message sent using the Messages API'
)
vonage_client.messaging.send(
  from: 'YOUR-INSTAGRAM-ID',
  to: 'USER-INSTAGRAM-ID',
  **message
)

[superchilled]

Feature is not yet GA. Don't merge.