firebase-ios-sdk

Fix building of objc protos (#1858)

All proto targets (objc, cpp, nanopb) now depend on the proto sources,
so they'll be rebuilt if the proto sources change.

Also updated Firestore/Protos/README.md to reflect recent changes.

Also update travis config to build the protos. (Previously, cpp and
nanopb were rebuilt, since they're used in the cmake build. But objc was
not since it's not actually depended on by anything in cmake.)