- require karma tests
 - temporarily turn off module transformation on airbnb-preset

 - spreading a non-iterable into a function call no longer works in babel 7

[Tests] pin included builds to LTS instead of “latest”, since node 9 is broken; use `nvm install-latest-npm` so new npm doesn’t break older node

[Dev Deps] update `eslint`, `eslint-config-airbnb`, `eslint-plugin-import`, `eslint-plugin-jsx-a11y`, `enzyme`, `mocha`, `sinon`, `babel-preset-airbnb`, `chai`, `airbnb-js-shims`

 - `node` `v7`/`v4.6`; drop `v5`
 - re-enable karma builds, but allow them to fail
 - correct before_script settings for karma tests
 - install appropriate react version, per `$REACT`
 - Ensure tests aren’t ran twice

Removes the short-lived tether dependency in react-dates in favor of just doing the thing internally

Since the primary use-case for react-dates is in a browser environment
it seems appropriate to run the test suite in a browser.

I've added configuration to run the test suite in Firefox via Karma
with the necessary adjustments to the Travis config. I've commented
the two places in the Karma config where I'm doing something odd in
order to make things work.