firebase-ios-sdk

Temporarily disable Messaging pod lints (#1277)

There's an unreleased change in Core that Messaging relies on, but Travis currently pulls in the publicly released version of Core causing the linting to fail. This should be re-enabled once either a) Core is released, or b) we support pointing to local pods for dependencies on Travis - whichever of the two come first.

An issue will be opened to re-enable the linting.