Support older versions of Stardog

[scardena]

Currently we are trying on our best effort to not push breaking changes, and keep features that work for previous stardog versions that don't work on newer and viceversa.
We noticed some stardog versions also break some of the tests for older pystardog versions.
We need to:

• Specify what Stardog versions are supported by a given pystardog version
• Run the pystardog test suite against multiple SD versions at the same time (so that we know nothing is broken. if there is something broken, we need to announce that we are pushing breaking changes)
• Run pystardog test suite weekly, so that we catch problems like license expiring, new stardog releases, and basically make sure nothing is broken. (notice that only latest stardog version is being pushed to docker hub, and we don't push older versions to it. we need to figure out a way on how to pull older versions of stardog so that we can run the circle test against them)
• Make sure pystardog tests can run against any supported stardog endpoint without leaving any dangling / leftover resources. it should be safe to run these tests without destroying any existing data.