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.

man/i3-input.man

@@ -26,7 +26,7 @@ Specify the path to the i3 IPC socket (it should not be necessary to use this
 option, i3-input will figure out the path on its own).
 
 -F <format>::
-Every occurence of "%s" in the <format> string is replaced by the user input,
+Every occurrence of "%s" in the <format> string is replaced by the user input,
 and the result is sent to i3 as a command. Default value is "%s".
 
 -l <limit>::