ENH: Create Travis Workflow

[kmuehlbauer]

The workflow to be created with TravisCI should:

• render the notebooks when in case of PR as well as MERGE/PUSH
• If MERGE/PUSH the rendered notebooks are updated in devel branch

[kmuehlbauer]

see #2 for first steps.

We can check the changed notebooks with:

changed=`git diff --name-only HEAD $(git merge-base HEAD base-branch) *.ipynb`

Then we can just render these. In this case we need to checkout the devel-branch, too.

[kmuehlbauer]

@heistermann
The travis workflow is now running with rerendering all notebooks. It will be a bit more of a work to get the things going. I leave the issue open to not forget.