Trim Travis config part 2

- Remove Win builds (temporarily). They're slow, allowed to fail,
  and therefore not useful as they are.
- Make the --unified part of the matrix build-only. (This can be
  swapped if --unified becomes the default)
- Only build 'no-engine' once, don't run any tests, but don't allow it
  to fail.

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