[WIP] Capistrano v3

This PR is NOT ready to merge, but here for comment and collaboration.

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So in playing around with getting Huginn deployed on digitalocean as per these instructions I seem to have endeavoured down the path of converting the Capfile to Capistrano v3.

This is a placeholder/discussion type issue, will link a PR for reference in a minute.

My current biggest issue (aside from learning Capistrano on the fly!) is that the .env file doesn't seem to be getting linked into place and/or used, so i get a Devise.secret_key was not set. error during deploy.

@cantino I see that you commited the original Capfile and the article. Are you up to scratch on v3 by chance?