CI: travis: revert to clang-7 for BoringSSL tests

Building BoringSSL with clang9 fails:
   https://travis-ci.com/github/haproxy/haproxy/jobs/298267505
   https://bugs.chromium.org/p/boringssl/issues/detail?id=323

Let's revert to clang-7 for now.
1 file changed