.travis.yml: pre-seed sub-modules for speed

A significant portion of the build time is spent initialising all the
sub-modules we use in the source tree. Often this is almost as long as
the build itself. By pre-seeding the .git/modules tree this will
hopefully improve things.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>

---
v2
  - use wget -O - | tar as suggested by agraf
.travis.yml: remove "make check" from main matrix

There are problems with unreliability in "make check" which still need
to be tracked down. As the tests are broadly the same for all targets if
added one explicit target to the matrix to run it. However this does
build all softmmu targets to ensure they at least "run"

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alexander Graf <agraf@suse.de>