New test runner

[tschaub]

This updates the CircleCI config to run tests on Firefox and Chrome. With this change, we no longer run tests on IE. This is something we can revisit if it becomes a problem.

If others agree with this change, we can update the branch settings to require the CircleCI job to pass (and remove the Travis requirement).

[ahocevar]

So nice! Please merge!

[tschaub]

Looks like some rendering tests are failing regularly on CircleCI's version of Firefox. See https://circleci.com/gh/openlayers/openlayers/23 for example.

The rendering tests are going to need a significant overhaul with the upcoming renderer changes (we will no longer have a single target canvas element for rendering). My plan was to separate out and simplify the rendering tests and run them with puppeteer. It would be nice to have cross-browser rendering and unit tests, but we could try simplifying and making the tests more reliable first and then rebuild cross-browser support if/when we feel like we need it.

[ahocevar]

For more reliable rendering tests, we'd also need to rework them to use larger reference images. For now, I'd say it's ok only test in Chrome.