docker: add gcovr to travis image

Useful for debugging if nothing else as the gcovr on the Travis images
are a little old.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/docker/dockerfiles/travis.docker

@@ -5,7 +5,7 @@ ENV LC_ALL en_US.UTF-8
 RUN cat /etc/apt/sources.list | sed "s/# deb-src/deb-src/" >> /etc/apt/sources.list
 RUN apt-get update
 RUN apt-get -y build-dep qemu
-RUN apt-get -y install device-tree-compiler 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 gcovr
 # Travis tools require PhantomJS / Neo4j / Maven accessible
 # in their PATH (QEMU build won't access them).
 ENV PATH /usr/local/phantomjs/bin:/usr/local/phantomjs:/usr/local/neo4j-3.2.7/bin:/usr/local/maven-3.5.2/bin:/usr/local/cmake-3.9.2/bin:/usr/local/clang-5.0.0/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin