Build against Ruby 2.7

[smudge]

Summary

This moves the repo onto Ruby 2.7.5 by default. I tried to make as few gem upgrades as I could in order to get onto 2.7, but it did require a move onto a newer capybara/selenium-webdriver, a newer rubocop, and a newer mini_racer (with libv8-node). I also removed poltergeist and got tests working with chromdriver in CI.

Other Information

Not much else! Stay tuned for some more updates after this!

/domain @Betterment/test_track_core @agirlnamedsophia /no-platform

[nanda-prbot]

Needs somebody from @Betterment/test_track_core and @agirlnamedsophia to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}

[nanda-prbot]

Needs somebody from @Betterment/test_track_core and @agirlnamedsophia to claim domain review

Use the shovel operator to claim, e.g.:

@myname << domain && platform

_{HOW TO: Claim a Review}