ci: install Asciidoctor in 'ci/install-dependencies.sh'

ci: install Asciidoctor in 'ci/install-dependencies.sh'

When our '.travis.yml' was split into several 'ci/*' scripts [1], the
installation of the 'asciidoctor' gem somehow ended up in
'ci/test-documentation.sh'.

Install it in 'ci/install-dependencies.sh', where we install other
dependencies of the Documentation build job as well (asciidoc,
xmlto).

[1] 657343a602 (travis-ci: move Travis CI code into dedicated scripts,
    2017-09-10)

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