Test against a build matrix

[andreasknoepfle]

With #153 we can see that it is sometimes useful to test not only against one but multiple versions of chrome, elixir, erlang & ghostscript. Maybe we can use a build matrix in the CI, so new versions can easily be added.

We could also use multiple versions of alpine images, since the chrome and ghostscript parts are pretty separate and there is always an update in major versions in a new alpine major version for chrome (and often ghostscript as well) and they reflect what is state of the art pretty well.