Releasing asciidoctor-maven-plugin v 1.5.1

Thanks all for all the encouraging comments.

About the tag you're right, I did not use the release plugin but I had the intention to create the tag later, the problem is that I was in a bit of a hurry yesterday and I did not want to delay the package any longer so that Paul can update the examples.

Now that I understand the whole process, at least I can keep the plugin aligned with asciidoctor.