travis: add boringssl build
Remains the issue of what revision to pick for tests.
In the ticket leading up to this one I suggested to not go with HEAD because sometimes breaking changes occur (which sometimes are reverted, sometimes not).
edit: Absent any better suggestion I'd like to repeat mine: Rewind to the latest tag for cocoapods, or any that matches
The debug option causes far too many warnings in boringssl headers. Valgrind triggers some false positive errors in thread-local data used by boringssl.