travis-ci: create the cache directory early in the build process

travis-ci: create the cache directory early in the build process

It seems that Travis CI creates the cache directory for us anyway,
even when a previous cache doesn't exist for the current build job.
Alas, this behavior is not explicitly documented, therefore we don't
rely on it and create the cache directory ourselves in those build
jobs that read/write cached data (currently only the prove state).

In the following commit we'll start to cache additional data in every
build job, and will access the cache much earlier in the build
process.

Therefore move creating the cache directory to 'ci/lib-travisci.sh' to
make sure that it exists at the very beginning of every build job.

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>