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.

src/commands.c

@@ -250,7 +250,7 @@ void cmd_criteria_match_windows(I3_CMD) {
                 DLOG("matches window!\n");
                 accept_match = true;
             } else {
-                DLOG("doesnt match\n");
+                DLOG("doesn't match\n");
                 FREE(current);
                 continue;
             }