docker: don't install device-tree-compiler build-deps in travis.docker

Installing the device-tree-compiler build-deps is a little extreme. We
only actually need the binary so include it with the other packages.
Suggested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20170725133425.436-6-alex.bennee@linaro.org>
Reviewed-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: NPhilippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: NFam Zheng <famz@redhat.com>

tests/docker/dockerfiles/travis.docker

 FROM quay.io/travisci/travis-ruby
 RUN apt-get update
 RUN apt-get -y build-dep qemu
-RUN apt-get -y build-dep device-tree-compiler
-RUN apt-get -y install python2.7 python-yaml dh-autoreconf gdb strace lsof net-tools
+RUN apt-get -y install device-tree-compiler python2.7 python-yaml dh-autoreconf gdb strace lsof net-tools
 ENV FEATURES pyyaml