[docs] Make autobuild regenerate docs builds when saving outside vagrant

[chimeno]

Currently when running autobuild.sh inside the vagrant and making docs changes outside of it, there is not build at all.

I've fixed with:

• upgrading watchdog from 0.8.3 to the git master branch in https://github.com/gorakhargosh/watchdog
• add a --debug-force-polling to autobuild.sh: watchmedo shell-command --debug-force-polling --patterns="*.rst" --ignore-pattern='_build/*' --recursive --command='make html; echo "Waiting for more changes..."'

I could make a PR, but I don't see when the watchdog library is installed in order to upgrade it.

[gasman]

I've never used autobuild.sh - I never knew it existed at all actually - and it looks like it was added before we had any formal / documented build environment, so it's very likely that watchdog isn't installed at all.

I don't think it should be added to Wagtail's own doc dependencies, because it's non-essential and adds new installation requirements (LibYAML). I think it would be most appropriate to add it to the provision.sh script in vagrant-wagtail-develop.