ci: stick with Asciidoctor v1.5.8 for now

ci: stick with Asciidoctor v1.5.8 for now

The recent release of Asciidoctor v2.0.0 broke our documentation
build job on Travis CI, where we 'gem install asciidoctor', which
always brings us the latest and (supposedly) greatest.  Alas, we are
not ready for that just yet, because it removed support for DocBook
4.5, and we have been requiring that particular DocBook version to
build 'user-manual.xml' with Asciidoctor, resulting in:

  ASCIIDOC user-manual.xml
  asciidoctor: FAILED: missing converter for backend 'docbook45'. Processing aborted.
  Use --trace for backtrace
  make[1]: *** [user-manual.xml] Error 1

Unfortunately, we can't simply switch to DocBook 5 right away, as
doing so leads to validation errors from 'xmlto', and working around
those leads to yet another errors... [1]

So let's stick with Asciidoctor v1.5.8 (latest stable release before
v2.0.0) in our documentation build job on Travis CI for now, until we
figure out how to deal with the fallout from Asciidoctor v2.0.0.

[1] https://public-inbox.org/git/20190324162131.GL4047@pobox.com/

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