Fix compatibility bugs with Firefox 25, 30, 31.

Shim `Array#find` and `Array#findIndex` if they don't match ES6 spec's
treatment of holes. (Tweak our `ToInteger` helper so that we can test
these methods before `Math.sign` has been shimmed.)

Broaden the permissible values of `Math.cosh`.

Fix an overly-specific test for `Symbol.iterator` (for compatibility,
we use `'@@iterator'` as the name of our iterator on Firefox, in the
absence of `Symbol.iterator`).

Fix a bad length property for `String.fromCodePoint`.

Closes: #256
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.
fixup, more platforms