Add more browsers to Sauce Labs

[vweevers]

Current: Chrome latest and FF latest. Add:

• Edge
• Safari
• IE11
• More Chrome or FF versions? Nah, they're evergreen.
• ..?

[vweevers]

Depending on which browsers we add, we might be able to remove the shim.

[vweevers]

Safari latest (11) passes tests.

[vweevers]

Edge latest (17) fails on:

• Array keys. Fixable by detecting array key support.
• Accessing transaction.error if transaction was completed (rather than aborted). Fixable.

[vweevers]

IE 11 fails on:

• Number.isNaN. Can use global isNaN instead.
• Use of Promise in levelup. Should include the same polyfill here.

[ralphtheninja]

Depending on which browsers we add, we might be able to remove the shim.

Which shim?

[vweevers]

https://github.com/Level/level.js/blob/b1c01a9a9c144d462c330fd09fd541baff6b5467/test/test.js#L5-L6