travis-ci: install packages in 'ci/install-dependencies.sh'

travis-ci: install packages in 'ci/install-dependencies.sh'

Ever since we started using Travis CI, we specified the list of
packages to install in '.travis.yml' via the APT addon.  While running
our builds on Travis CI's container-based infrastructure we didn't
have another choice, because that environment didn't support 'sudo',
and thus we didn't have permission to install packages ourselves.  With
the switch to the VM-based infrastructure in the previous patch we do
get a working 'sudo', so we can install packages by running 'sudo
apt-get -y install ...' as well.

Let's make use of this and install necessary packages in
'ci/install-dependencies.sh', so all the dependencies (i.e. both
packages and "non-packages" (P4 and Git-LFS)) are handled in the same
file.  Install gcc-8 only in the 'linux-gcc' build job; so far it has
been unnecessarily installed in the 'linux-clang' build job as well.
Print the versions of P4 and Git-LFS conditionally, i.e. only when
they have been installed; with this change even the static analysis
and documentation build jobs start using 'ci/install-dependencies.sh'
to install packages, and neither of these two build jobs depend on and
thus install those.

This change will presumably be beneficial for the upcoming Azure
Pipelines integration [1]: preliminary versions of that patch series
run a couple of 'apt-get' commands to install the necessary packages
before running 'ci/install-dependencies.sh', but with this patch it
will be sufficient to run only 'ci/install-dependencies.sh'.

[1] https://public-inbox.org/git/1a22efe849d6da79f2c639c62a1483361a130238.1539598316.git.gitgitgadget@gmail.com/

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