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>