ci: move the existing test under a jobs section, so we can add others later
ci: move the existing test under a jobs section, so we can add others later

In the next commit Pierre will start adding tests for the docker build, and this
lays out the structure for doing that.
No functional changes.

The relevant TravisCI docs that motivates moving under a jobs section is
https://docs.travis-ci.com/user/build-matrix/

> There are two ways to specify multiple parallel jobs (what we call the build
> matrix) with a single .travis.yml configuration file:
>
> * combine a language-and-environment dependent set of configuration options to
>   automatically create a matrix of all possible combinations. This is called
>   matrix expansion. For example, the following configuration produces a build
>   matrix that expands to 8 individual (2 * 2 * 2) jobs
>   [...]
>
> * specify the exact combination of configurations you want in jobs.include.
>   For example, if not all of those combinations are interesting, you can
>   specify just the combinations you want

ether/etherpad-lite,ether/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,aptivate/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,storytouch/etherpad-lite,ether/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite