.travis.yml: run make check for all matrix targets

We only ran make check once before it used to be an unreliable target.
It was only a stop gap measure and we should be able to revert it now.
This also stops us needing a large all-MMU build.

We disable "make check" for a couple of the extra config targets which
are currently broken.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Tested-by: David Gibson <david@gibson.dropbear.id.au>
