Use '\' to chain arguments in doc publication pipeline

[abelsromero]

Completing #708

From the error in CI we can infer the arguments are interpreted as independent statements.

/home/runner/work/_temp/f899399a-9ce7-4712-b5d6-6c9ed60e3894.sh: line 3: -Dorg.ajoberstar.grgit.auth.username=: command not found

[abelsromero]

I am proceeding since I was granted permission some time ago 🤞

[abelsromero]

Publication is not working

Task ':gh-pages:publishDocs' is not up-to-date because:
  Task has not declared any outputs despite executing actions.
not-on-travis-ci is not a match. Push skipped.

Seems we need to update how we check for the branch and check we are in CI

asciidoctor-gradle-plugin/docs/gh-pages/build.gradle

Line 6 in 131bc74

currentBranch = System.getenv('TRAVIS_BRANCH') ?: 'not-on-travis-ci'