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.

libi3/format_placeholders.c

@@ -16,7 +16,7 @@
 #endif
 
 /*
- * Replaces occurences of the defined placeholders in the format string.
+ * Replaces occurrences of the defined placeholders in the format string.
  *
  */
 char *format_placeholders(char *format, placeholder_t *placeholders, int num) {