Fixed Travis support

No need to clone concrete5 on Travis, ping @aembler.

I created the original Travis support for the tests. We needed to clone concrete5 then, as it was in a separate repository. As this is no more the case no clone is needed (Travis clones the project itself).

At the moment the install fails due to a bug reported in concrete5/concrete5-cli#15, for a successful build see https://travis-ci.org/worldskills/concrete5/builds/14189624.