Run tests in multiple simulators

[brockboland]

Travis currently runs tests in iOS, tvOS, and macOS, but I'd like to have it run those tests on multiple versions of iOS, if possible.

I'm sure we can accomplish this with scan but I'm not exactly sure how to do so. The destination option looks like the one we want, but the docs aren't especially helpful:

-d, --destination [VALUE] Use only if you're a pro, use the other options instead (SCAN_DESTINATION)

[brockboland]

Actually, this might be the better option here:

-k, --sdk STRING     The SDK that should be used for building the application (SCAN_SDK)