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.  (Be sure to do this test after `Number` has been
shimmed, and eliminate a premature use of `Math.sign`.)

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`).

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.  Be
careful when shimming iterators.
SauceLabs/Travis integration.

We are currently failing IE9 and iPhone OS 5.1 because the
`promises-es6-tests` package requires strict mode support.
Limit the number of browser configurations tested by sauce by default.

We seem to be hitting job limits, probably because I've been
testing this hard all day.  Also make passing the sauce tests
optional for now.