Revert "Revert "Bug 1039966 - Update gaiatest marionette_client to 0.8.1 and dependency moztest to >= 0.6""

This reverts commit d15e133c0bac9f22218991ea2de157387ece792c.
Merge pull request #22324 from zacc/bug_1039966

Revert "Revert "Bug 1039966 - Update gaiatest marionette_client to 0.8.1...