Check that server hasn't been upgraded on each ajax request

If so, force the client to refresh the page.

This works with a npm postinstall hook, which writes the msSinceEpoch date into build.revision on heroku.
App checks that client has a 'build.revision'- cookie which matches the build.revision file content.
Cookie is set when an entrypoint html page is loaded (e.g. public.html).

(Turned out that accessing e.g. the current git sha was impossible in heroku)
Merge pull request #65 from kavi-fi/client-server-version-mismatch

Check that server hasn't been upgraded on each ajax request