Update chai configuration.

The old `chai.Assertion...` style has been deprecated in chai 1.9.1.
What's more, it triggers warnings when used, and those warnings use
`console.warn`... which isn't supported on IE <= 8.  So this was causing
testling failures on IE 8 which weren't even our fault.

This should return us to all-green on testling.

We also bumped the version of the promises-aplus-tests dependency
while we were at it.  That synchronizes the version of mocha
expected for promises-aplus-tests with the version we expect.
IE8 support: Don't require Object.create for Promise implementation.

Fixes: #236
Remove `Object.getOwnPropertyDescriptors` per spec.

Fixes #234
Merge pull request #238 from cscott/gh-234

Remove `Object.getOwnPropertyDescriptors` per spec.
Merge pull request #237 from cscott/fix-chai

Fix IE8 issues.
Update CHANGELOG.