Run travis pod spec lint testing for Xcode 9

[paulb777]

The release 5.13.0 release testing discovered two Xcode 9 pod spec lint errors that don't show up in Xcode 10. We need to add Xcode 9 lint checking to regular Travis regression testing.

• Fix GoogleUtilities nullability regressions #2079 fixed introduced nullability warnings in GoogleUtilities
• Fix tablet layout for in-app messaging #2032 introduced Xcode 9 build errors for InAppMessaging

cc: @Corrob

[Corrob]

Interesting. Should pod spec linting run against both Xcode 9 and 10?

[paulb777]

Yep that's what this issue is to track - we should add another travis stage to do so.

It should be less important for non-source pods except for ones with resources.