Tweak version generation.

Instead of relying on sed to replace a VERSION token, we generate
src/start.js, which defines the internal `d3` variable with the current
version derived from package.json.

In addition, all file generators (component, locale, start) have been
moved to bin/.
Remove redundant VERSION from test sandbox.

This is no longer needed due to
22b744b7e60122bee5abb82642ecb4feefa873d0.