Rather than relying on the release version being passed in as config,
build the current release version (as read from `package.json`) into the
bundle.

We use browserify-versionify for this, rather than simply requiring
`package.json`, as our current build toolchain does not support any kind
of tree-shaking, so `require('package.json')` would pull the whole of
the `package.json` file into the bundle.