ci: install 'libsvn-perl' instead of 'git-svn'

ci: install 'libsvn-perl' instead of 'git-svn'

Since e7e9f5e7a1 (travis-ci: enable Git SVN tests t91xx on Linux,
2016-05-19) some of our Travis CI build jobs install the 'git-svn'
package, because it was a convenient way to install its dependencies,
which are necessary to run our 'git-svn' tests (we don't actually need
the 'git-svn' package itself).  However, from those dependencies,
namely the 'libsvn-perl', 'libyaml-perl', and 'libterm-readkey-perl'
packages, only 'libsvn-perl' is necessary to run those tests, the
others arent, not even to fulfill some prereqs.

So update 'ci/install-dependencies.sh' to install only 'libsvn-perl'
instead of 'git-svn' and its additional dependencies.

Note that this change has more important implications than merely not
installing three unnecessary packages, as it keeps our builds working
with Travis CI's Xenial images.  In our '.travis.yml' we never
explicitly specified which Linux image we want to use to run our Linux
build jobs, and so far they have been run on the default Ubuntu 14.04
Trusty image.  However, 14.04 just reached its EOL, and Travis CI has
already began the transition to use 16.04 Xenial as the default Linux
build environment [1].  Alas, our Linux Clang and GCC build jobs can't
simply 'apt-get install git-svn' in the current Xenial images [2],
like they did in the Trusty images, and, consequently, fail.
Installing only 'libsvn-perl' avoids this issue, while the 'git svn'
tests are still run as they should.

[1] https://blog.travis-ci.com/2019-04-15-xenial-default-build-environment

[2] 'apt-get install git-svn' in the Xenial image fails with:

      The following packages have unmet dependencies:
       git-svn : Depends: git (< 1:2.7.4-.)
      E: Unable to correct problems, you have held broken packages.

    The reason is that both the Trusty and Xenial images contain the
    'git' package installed from 'ppa:git-core/ppa', so it's
    considerably newer than the 'git' package in the corresponding
    standard Ubuntu package repositories.  The difference is that the
    Trusty image still contains these third-party apt repositories, so
    the 'git-svn' package was installed from the same PPA, and its
    version matched the version of the already installed 'git'
    package.  In the Xenial image, however, these third-party
    apt-repositories are removed (to reduce the risk of unrelated
    interference and faster 'apt-get update') [3], and the version of
    the 'git-svn' package coming from the standard Ubuntu package
    repositories doesn't match the much more recent version of the
    'git' package installed from the PPA, resulting in this dependecy
    error.

    Adding back the 'ppa:git-core/ppa' package repository would solve
    this dependency issue as well, but since the troublesome package
    happens to be unnecessary, not installing it in the first place is
    better.

[3] https://docs.travis-ci.com/user/reference/xenial/#third-party-apt-repositories-removed

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