docker: add debug tools to travis.docker

When a test fails/hangs you don't want the hassle of getting the debug
tools installed. Lets install them on our image by default so we can
debug when we need to.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>