.travis.yml: minor facelift

Apparently trusty image has newer clang, there is no need to pull
clang-3.9 packages. It's clang-5.0.0, installation is a bit quirky,
as it fails to compile for example strcmp(s,"-") without warning,
and complains about unreferred -I flags. But it's argued that benefits
of exercising newer sanitizer outweights the inconvenience of
additional -D__NO_STRING_INLINE and -Wno-unused-command-line-argument.

Also pull golang when actually needed.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6185)