fix(runner): prefer sauce to hosted driver when sauceKey is present

Sauce Connect sets up a local Selenium relay at localhost:4445 by
default. Allow users to adjust the `seleniumAddress` to point at this
secure relay instead of the default `ondemand.saucelabs.com` host while
still using the `sauce` driver provider.

Previously, runner would load the `remote` driver provider if
`seleniumAddress` was configured, even if Sauce credentials were also
configured.

For reasons not clear to me, on Travis VMs, the Sauce Connect relay is 10x more reliable than `ondemand.saucelabs.com`. It might be worth testing whether this helps with protractor's test suite oddities (#1051 and #1052)