Make the check target build ol.js

With a [commit](https://github.com/openlayers/ol3/commit/e9c6e58663772699712ba922f22ae31a922c901a) that was recently merged the `check` target just runs the linters and the tests but doesn't compile the ol3 code anymore. The `check` target is the target we [tell](https://github.com/openlayers/ol3/blob/master/CONTRIBUTING.md#running-the-check-target) people to use before submitting PRs, so I think that target should check that the ol3 code compiles.