travis-ci: save prove state for the 32 bit Linux build

travis-ci: save prove state for the 32 bit Linux build

This change follows suit of 6272ed319 (travis-ci: run previously
failed tests first, then slowest to fastest, 2016-01-26), which did
this for the Linux and OSX build jobs.  Travis CI build jobs run the
tests parallel, which is sligtly faster when tests are run in slowest
to fastest order, shortening the overall runtime of this build job by
about a minute / 10%.

Note, that the 32 bit Linux build job runs the tests suite in a Docker
container and we have to share the Travis CI cache directory with the
container as a second volume.  Otherwise we couldn't use a symlink
pointing to the prove state file in the cache directory, because
that's outside of the directory hierarchy accessible from within the
container.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Reviewed-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>