travis-ci: fix running P4 and Git LFS tests in Linux build jobs

travis-ci: fix running P4 and Git LFS tests in Linux build jobs

Linux build jobs on Travis CI skip the P4 and Git LFS tests since
commit 657343a60 (travis-ci: move Travis CI code into dedicated
scripts, 2017-09-10), claiming there are no P4 or Git LFS installed.

The reason is that P4 and Git LFS binaries are not installed to a
directory in the default $PATH, but their directories are prepended to
$PATH.  This worked just fine before said commit, because $PATH was
set in a scriptlet embedded in our '.travis.yml', thus its new value
was visible during the rest of the build job.  However, after these
embedded scriptlets were moved into dedicated scripts executed in
separate shell processes, any variable set in one of those scripts is
only visible in that single script but not in any of the others.  In
this case, 'ci/install-dependencies.sh' downloads P4 and Git LFS and
modifies $PATH, but to no effect, because 'ci/run-tests.sh' only sees
Travis CI's default $PATH.

Move adjusting $PATH to 'ci/lib-travisci.sh', which is sourced in all
other 'ci/' scripts, so all those scripts will see the updated $PATH
value.

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