Add litmus to testing pipeline

[patrickjahns]

To ensure proper webdav compatibility at all time - we want to also test litmus when the storage backend has been changed.

[ownclouders]

GitMate.io thinks the contributor most likely able to help you is @DeepDiver1975.

Possibly related issues are #49 (Add unit tests), #38 (Add makefile), #28 (Test against master again), #74 (run testing against nightly tarball), and #65 (Manual Test for Files Primary S3 ).

[patrickjahns]

Will also be handled in https://github.com/owncloud/litmus-testing

needs to be decided if it will be added to the app pipeline