Deploy mkdocs when merging to master

[marcusholl]

Only changes to master and PRs starts the travis job.
On PR Maven tests and mkdocs build for warnings and error check is done.
On change on master Maven is not run and the documentation is built
and deployed using a deploy key. The encrypted deploy key is not part of
this commit, it needs to be provided by an admin for this repository.

[marcusholl]

@o-liver You triggered deployment of changes on master branch initially.

Some questions:

• can you have a look into the rebased and adapted version here, please?
• How can we avoid gh-pages updates for master branches in forks?
• What is the story behind deploy-key.enc file. I cannot find the file in the sources.

[o-liver]

I presume this lets you spread the if then else over a few lines. Is that correct?

[marcusholl]

yes

[o-liver]

Are you sure you don't have to make this file executable before?

[marcusholl]

I adjusted the file mode for gh-pages-deploy to 755. I gets checked out from git with the executable flag already set.

But I see now something else: should be ./gh-pages-deploy rather than simply gh-pages-deploy.

[o-liver]

• can you have a look into the rebased and adapted version here, please?
  Looks good.

• How can we avoid gh-pages updates for master branches in forks?
  You could check the TRAVIS_REPO_SLUG environment variable.
  Or you could define an environment variable only available to the non-fork as described here

• What is the story behind deploy-key.enc file. I cannot find the file in the sources.
  The procedure is to generate a key pair and add the public key to the repository on github while encrypting the private key with the travis cli and adding it to the sources.

[marcusholl]

Travis build number 491 was fine, but did not report back.
This build actually did not trigger a deployment (since PR must not trigger a deployment). But there is also PR492, which was not a PR and for that the matching branch was updated accordingly (marcusholl/experimental) did in fact deploy.

[alxsap]

looks good