Fix regression in Map/Set shim in latest Chrome betas.

The `defineProperties` method doesn't work if we are using
`Symbol.iterator` as the name for the iterator method.  Only newer
browsers define enough of `Symbol` to encounter this problem.
SauceLabs/Travis integration.

We are currently failing IE9 and iPhone OS 5.1 because the
`promises-es6-tests` package requires strict mode support.

Firefox >= 25 is also failing due to some issue with Array.@@iterator
and sparse arrays.  See gh #256.