[maven-release-plugin] copy for tag asciidoctor-maven-plugin-2.0.0

update version to 1.6.0 + fix javadocs + remove java7

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-2.0.0-RC.1

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-1.5.8

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-1.5.7.1

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-1.5.7

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-1.5.6

[maven-release-plugin] copy for tag asciidoctor-maven-plugin-1.5.5

update version number in code snippet

prepare release asciidoctor-maven-plugin-1.5.2.1