* refactor the `readMetaData()` function
* add a couple new test cases
* test: restore "build" prefix
Fixes Node.js <= v6
* add default values for `npm`/`docker` get files functions
Makes the logic simpler, since we don't have to check for existence.
* throw an error when missing start/now-start or server.js
* pass in entire `readMetaData()` result to Now.create()
This avoids us reading these files from the filesystem again