Fix extremely large number of deprecations and add GitHub actions

[hut8]

This addresses #481

Summary of changes:

• To begin with, obviously I needed to set up a local dev environment. The version of Ruby that we are using won't build via rvm on Debian Buster, which I am using. So I began by upgrading Ruby to 2.4, which is the earlier version that installs without issue.
• Of course, the whole point of CI in this case was just to run unit tests. So I started with that:
• A large number of them failed. Rubygems itself changed a bit of its core, and thanks to the PR by @20goto10, I fixed that.
• @joshjordan did a great job with tests, as always, for the code he wrote. Stripe changed their input iframe, though, so I had to fix the tests to accommodate the changes.
• When running the tests, bower failed every time it was invoked. Oh, jeez. Now this is a can of worms. I began by installing the same versions that were present on production of both NPM and Node and trying again. No luck, but different errors. I scp'd the production node_modules directory locally to investigate. All of the dependencies were basically completely different. This is the problem, see the next bullet point for the solution.
• NPM basically used to be a "wild west" with installs. NPM prior to 5 did not have a package-lock.json, so you could install a specific-ish version for your dependencies, but then those dependencies would all end up installing whatever was at the highest end of the acceptable semver range. I had already tried running this with the LTS node and npm but that did not work, so I downgraded to node 4.2.6 (from production) and then installed the earliest version of npm (5) that supports package-lock.json. Well, of course, that doesn't run on 4.2, you need 4.4 at least. So then I upgraded node to 4.4 with NPM 5. Then I ran npm i --package-lock-only on my local copy of the current production deployment. That is where the package-lock.json came from, which is very important. Now we will end up deploying consistent versions every time.
• Many of the dependencies did not like Ruby 2.4, so I upgraded those.
• rspec-core seems to have made a change that required me to move @example into a block parameter.
• Several places in the code, including in our own Gemfile, the git://github.com/ protocol/server was used, and that doesn't exist anymore. I had to fix these to even install it.
• I had several errors (which frankly I cannot remember) with the Selenium driver, which was extremely old and also didn't play well with the slightly newer dependencies. I replaced it with the headless chrome driver, which doesn't look too hard to get working on CI.
• I fixed another issue with Stripe tests which I cannot explain but now it works.

GitHub Actions CI Setup

Then after doing the aforementioned yak shaving, I added one file, verify.yaml, which runs rspec. Now CI works.

I also updated the README to add a deprecation notice, and updated our build status badge.

[hut8]

@sheerun You wanna review too?

[hut8]

@joshjordan said LGTM; is AFK RN.

[sheerun]

@hut8 Sad, but true. Thank you for putting it to an end gracefully