.travis.yml: Run make check for all targets, not just some

ed173cb ".travis.yml: remove "make check" from main matrix" stopped running
make check for all the Travis build targets for various reasons.  It
continued to run make check on one Travis build, which builds for a big
list of all (? nearly all) our supported softmmu targets.

Unfortunately, due to a spacing / quoting error it only actually builds for
the alpha, arm, aarch64 and cris targets.  Specifically, the list of
targets is split over several lines.  Even with YAML folding, this will
leave spaces in the list, meaning $TARGETS won't have the value we need.

I had a look at the YAML spec and I couldn't quickly see a way of splitting
the list so that it doesn't end up with spaces, so this patch fixes the
problem by putting the whole list on one huge line.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
