Fix travis clang-3.9 builds

Something environmental changed in travis so that it started preferring
the ubuntu clang-3.9 version instead of the llvm.org one. This breaks the
sanitiser based builds. This change forces travis to de-prioritise the
ubuntu clang packages.

[extended tests]

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3759)