feat(build): add support for publishing api docs and showcase using Travis

[christophercr]

Closes #282

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#-commit-message-guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[X] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Build related changes
[X] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

We generate our docs but don't publish them.

Issue Number: #282

What is the new behavior?

We publish the API docs and the showcase on GitHub pages.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information

After looking at different alternatives, we decided to follow this approach:
https://gist.github.com/douglasduteil/5525750

[coveralls]

Coverage decreased (-0.01%) to 94.247% when pulling f38ff86 on christophercr:feature/ghdeploy into 1c310b6 on NationalBankBelgium:master.