.travis.yml: migrate to using docker for testing
.travis.yml: migrate to using docker for testing

This uses the customly build rauc/testing docker image for
running tests.

This aims to eliminate the current drawbacks and pains we see when
testing with Travis on a more recent platform as ubuntu trusty is.

Signed-off-by: Enrico Joerns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

ejoerns/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,jluebbe/rauc,ejoerns/rauc,rauc/rauc