travis-ci: Use "slim" Debian image for testing

Documentation and similar files are stripped from this image, making
it quicker to install.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

Closes: #600
Approved by: cgwalters

.travis.yml

@@ -4,7 +4,7 @@ sudo: required
 
 env:
   - ci_distro=ubuntu ci_suite=trusty
-  - ci_docker=debian:jessie ci_distro=debian ci_suite=jessie
+  - ci_docker=debian:jessie-slim ci_distro=debian ci_suite=jessie
   - ci_docker=ubuntu:xenial ci_distro=ubuntu ci_suite=xenial
 
 script: