Update clang-format version to latest on Travis

[bstpierr]

Homebrew is now serving up clang-format v8.

[bstpierr]

It actually looks like some of the style rules have changed between version 7 and 8. See this commit on my other PR. clang-format 8 had fixes the spacing and long line, but clang-format 7 saw those as wrong.

Do we have a plan for how to handle style discrepancies between clang-format versions?

[ryanwilson]

It looks like Travis is still pulling from 7: https://travis-ci.org/firebase/firebase-ios-sdk/jobs/458598973#L684

Let's set up a discussion to see what the strategy is, if we should always move to latest or not.

[bstpierr]

From the documentation it seems like they just use what's on their disk image, but it can run brew update if we would like. In the past we just allowed it to stay old, but update the script like I did in this PR. This allows us to update the version on our machines ahead of Travis and fix style issues before Travis breaks us.

[bstpierr]

Oops, didn't mean to close this PR. Reopening it.

[paulb777]

Update instructions at https://github.com/firebase/firebase-ios-sdk#code-formatting

We should probably wait until after M40 for bumping the clang-format version since likely it would force all of open source to release.

[paulb777]

Updating clang-format turned out to be a natural follow-on of updating travis to Xcode 10.1 and I did it in #2222.