Deploying a snapshot will update the cache/version number

This PR creates a new version whenever we deploy a snapshot using the `make deployint SNAPSHOT=***`, `make deployprod SNAPSHOT=****` or `./scripts/deploysnapshot.sh int/prod ****` commands because that's usually what we want.

Note that we need to have the same version number on each instance of a given staging because otherwise, our offline/appcache mechanism will fail.

@procrastinatio Does this make sense?
Make appcache lacks apache/app.conf rebuilding. The above rebuilds only the files needed on version change: index.html, mobile.html (both for prd and src), geoadmin.appcache and apache/app.conf, so it's pretty fast (no minifying, the jsbuild, etc).