* Integrate Test262
Introduce a GNU Make target for retrieving TC-39's Test262 suite and
validating parsing of the files it contains. Interpret each file as a
parser test in accordance with that project's `INTERPRETING.md`
document. Allow for the specification of allowed failures via a
"whitelist" file so that the test suite may help prevent regressions in
this project in situations where this project has known bugs. Initialize
the "whitelist" file with a listing of all tests that are currently
failing. Extend the continuous integration environment's configuration
to automatically run these tests.
* use graceful-fs and latest yarn on travis
