Force Firefox to start up with an empty page

Otherwise it could happen that some Mozilla page gets shown which has a
minimal size larger than the 786px we're requesting.  And the screenshot
will span that entire page even if the window is smaller, resulting in a
failure to adjust screenshot size.

See http://kb.mozillazine.org/Browser.startup.homepage_override.mstone
and http://kb.mozillazine.org/Browser.startup.page for details.

Just in case, we also include the docker image digests in the travis build
log, to increase chances of reproducing what we get there.

.travis.yml

@@ -7,6 +7,7 @@ services:
 before_script:
   - docker pull selenium/standalone-firefox:2.48.2
   - docker pull selenium/standalone-chrome:2.48.2
+  - docker images --no-trunc
 script:
   - npm test
   - dockers/Screenshotter/screenshotter.sh --verify