Action Cable: Tune Sauce Labs tests

[javan]

• Slims down the number of browser versions we test to improve overall test stability. We're limited by the number of VMs we can spin up on Sauce Labs so concurrent test runs could time out waiting for an available VM. Their mobile VMs are somewhat unstable (https://saucelabs.com/beta/tests/dc09ac05cdf34985877c1e64c1dd3240) so starting less of them may help overall.
• Locks to a known stable version of Faye to avoid failures like: https://saucelabs.com/beta/tests/69a71b9a848c49a49c4bf4582f063d95

/cc @maclover7 @jeremy