Upgrade dependencies

Some tidying up and updating of dependencies, specifically express, to fix some security issues with express' dependencies.

- Upgraded express to 3.17.5
- Removed regexp-quote as it wasn't used anywhere
- Removed the unnecessary assert dependency
- Moved the glob module into dev dependencies
- Updated app.js to use new express syntax

What I haven't done is fix the deprecation warnings about multipart modules, since there will probably be more changes related to that once https://github.com/mysociety/popit-api/pull/79 is merged in.

Part of https://github.com/mysociety/popit/issues/638