Temporarily disable the explicit enable-asan build

The explicit enable-asan build fails in the memleak test for unknown
reasons. Therefore we disable it temporarily to get a green Travis.

Other builds that use -fsanitize=address in Travis seem to pass.

[extended tests]

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/10689)
