Package seed should only download the packages that are compatible

[StephenHodgson]

XRTK - Mixed Reality Toolkit Bug Report

Describe the bug

When the package seed imports the latest releases from each xrtk package there may not have been an update to dependencies that will end up causing compiler errors.

Ideally the seed package should only download the latest confirmed working configuration.

This should go away with #85 but in the mean time is an issue we need to be aware of.

Easiest workaround, is to just move the xrtk.core to the previous version.

Expected behavior

Seed .unitypackge should only download the validated configuration.

Actual behavior

Seed is downloading the latest tags from each package even though the lastest might not support each others dependencies.