Trim the Travis config

- Remove no-asm. We've got to cut something, and this is at least
  partially covered by the sanitizer builds.
- Remove enable-crypto-mdebug from sanitizer
  builds. enable-crypto-mdebug has been shown to catch some static
  initialization bugs that the standard leak sanitizer can't so
  perhaps it has _some_ value; but we shouldn't let the two compete.

Reviewed-by: Richard Levitte <levitte@openssl.org>