Trim Travis config part 3

- Only build & test two configurations. Make all the
  other build variants buildonly on gcc (clang on osx).
- Don't build with default clang at all on linux.
- Only use gcc-5 and clang-3.6 for the sanitizer builds. Re-running
  e.g. CONFIG_OPTS="shared" with them seems redundant.

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