travis: check spelling of binaries and manpages, use docker

We now build a docker base container based on debian sid (where the very
latest packages are available). That base container is updated once a
month, or whenever travis-build.Dockerfile or debian/control change, but
re-used for subsequent travis runs. While the initial build might take
up to 15 minutes, subsequent builds typically run in a minute or two.

All the different steps that we run on travis are now factored into
separate scripts in the travis/ directory.

Switching to docker should also help with issue #2174.

travis/check-formatting.sh

@@ -0,0 +1,2 @@
+#!/bin/sh
+clang-format-3.5 -i $(find . -name "*.[ch]" | tr '\n' ' ') && git diff --exit-code || (echo 'Code was not formatted using clang-format!'; false)