travis: Use Clang 4.0 instead of 3.9 due to va_start() warnings

travis: Use Clang 4.0 instead of 3.9 due to va_start() warnings

This is a follow up on the issue documented in the previous commit.

To build with -Werror and Clang 3.9 we'd have to change all enum arguments
that are used as last argument before ... to e.g. u_int, which affects
quite a lot of places (crypto-factory, MODP_CUSTOM constructors, auth-cfg,
bus, vici-builder, vici-message).  Besides that it doesn't look as nice
it also seems a bit too much hassle just to cater to the whims of a
particular version of one compiler, so we just don't build with that
version on Travis and use 4.0 instead.