Fixes missing ptrs in docs.

Revert to Apple Bazel rules 0.12.0 as 0.13.0 no longer supports the d…

…efault test host.

Updates Travis to Xcode10 and updates Swift version from 4.0 to 4.2. (g…

…oogle#93)

* Updates Travis to Xcode10 and updates Swift version from 4.0 to 4.2.

* Removes `-wmo` copts flag from `PromisesTestHelpers` target due to bug when building against Xcode 10.