Travis build matrix adjustments

- do not exclude all clang builds on Linux
- exclude the constantly timeouting -fsanitize=address build on OS/X
- drop some mostly duplicate builds
- change the base linux distro to Bionic
- drop sudo as that is no longer needed - always on
- drop -D__NO_STRING_INLINES where not needed
- memleak test is not working with old clang

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11468)
