Add CI

[fscheiner]

The changes in this PR will implement the creation and deployment of the HTML files with Travis-CI. All HTML files created from AsciiDoc files will be removed with this PR, they will be deployed to another branch (see below) together with the other "static" (HTML) files (404.html, API docs, GT JIRA archive and exemplary files) upon build.

This will use a deploy key enabled over at gridcf/gct-docs and stored encrypted in the repo.

Currently Travis-CI builds are disabled for gridcf/gct-docs. We can re-enable these when:

• [x] the gridcf/gct-docs:gh-pages branch was renamed to adoc (staying the default branch)
• [x] a new empty gridcf/gct-docs:gh-pages branch was created

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A successful build configured for my fork can be seen on https://app.travis-ci.com/github/fscheiner/gct-docs/builds/242884858 with the target branch here: https://github.com/fscheiner/gct-docs/tree/gh-pages-html-only

[fscheiner]

@matyasselmeci, @msalle, @ellert: Well, in the end this didn't run as smooth as I assumed, but with the recent commits (https://github.com/gridcf/gct-docs/commit/11d4eff576249f3773c0a14379cdceee5eee326a being important and https://github.com/gridcf/gct-docs/commit/ec20e78876f2126d968a7af63ecd43cb6a66e1d0 needed to make:

Authenticate deployment
[...]
$ ssh -i ~/.dpl/deploy_key -T git@github.com 2>&1 | grep successful > /dev/null

...work successfully) it finally works.

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So from now on, changes to the ADOC files are made in the "adoc" branch. Static (HTML) files like 404.html, API docs, GT JIRA archive and exemplary files also belong here. Every file/dir not excluded by .rsync-exclude will by rsynced to html/ and from there deployed to the gh-pages branch (incl. keeping the history) for every push to the "adoc" branch. The old history of the branch originally named "gh-pages" is still available in the history of the "adoc" branch.