Improve Asciidoctor version number management

[dduportal]

So the Asciidoctor's version is currently written in 3 locations into the repository:

• Into the Makefile as a an expected single entrypoint (it is propagated to the docker build command as build argument + it is passed as environment variable to the test suite).
• Into the Dockerfile as default value for the ARG ASCIIDOCTOR_VERSION directive:
  • This is overwritten during the Travis Build
  • But the DockerHub was not providing parameterized build capability when the new build process was done.
• Into the README.adoc as sharply caught by @mojavelinux in Asciidoctor to 1.5.7.1 #71

The goal of this issue is to track the work related to having a single location.

PS: Might be linked with #54