Configure project to use Travis

[alpgarcia]

@jsmanrique proposed the following check:

for each <collection_name>/<panel_file_name>.json there must be a docs/_<collection_name>/<panel_file_name>.md and docs/assets/images/<collection_name>/<panel_file_name>.jpg

After reading this I think you are the right person to review #304. @sduenas do you know if this check could be hooked somewhere?

@sduenas said:

With Travis would be possible. It would be like a check that is run by the tests - in the same way it is done with flake8 in other repositories.

So we need to configure Travis and add that check.