Agent-based version of Percy Capybara

[anaulin]

Welcome to the new version percy-capybara, now @percy/agent-based! 🎆

Aside from the substantially smaller implementation (thanks, @percy/agent), the main changes in this PR are:

• percy-capybara now ships with a vendorized copy of percy-agent.js. This PR includes the necessary npm script and instructions on how to update this vendorized file.
• Testing is substantially different. Following the pattern we've established in our other SDKs, the testing of this package is largely integration-level testing, via posting live snapshots to Percy. I've opted to remove the local test cases that existed in this package because they seemed to be testing cases that are tested already in the @percy/agent package (and also in smoketests).
• Running tests for this package is now best done via bundle exec rake snapshots, which will wrap rspec execution in percy --exec.

In terms of compatibility, after upgrading to this version, users will need to:

• Replace calls to Percy::Capybara.snapshot(page, maybe_options) with Percy.snapshot(page, maybe_options).
• The options object remains the same, and should be fully backwards-compatible. The include_iframes option will be ignored, but iframes will work fine without it (thanks, @percy/agent).
• Delete calls to Percy::Capybara.initialize_build and Percy::Capybara.finalize_build, and any other Percy::Capybara... calls they might have. This includes setting loaders. Loaders are no longer a thing. 😌
• Wrap their test-runner calls in percy exec --

In the "what is Ruby, even" category, I'm sure you'll have comments on style, and on my use of return (yay? nay?). I am uncertain if the file/directory structure that we have now in this package follows Ruby gem idioms. In particular, percy/capybara.rb now contains only code for the Percy module. Not sure if that's kosher or not.

@fotinakis for review, @Robdel12 fyi

[anaulin]

The CI build on this PR is now green, module getting approval on the visual diff in: https://percy.io/percy/percy-capybara/builds/1504564

[anaulin]

I blame node for messing up the nice code deletion stats in this PR.

[anaulin]

@fotinakis i believe i've addressed all your comments on this code, please have another look

[anaulin]

The branch I created before starting this PR, with the changes at master for percy-capybara version 3.2.0 are on branch: pre-agent-v320 (https://github.com/percy/percy-capybara/tree/pre-agent-v320)