Update New Spring Starter Wizard

There's now an option on 'start.spring.io' to choose spring boot version. We should surface that option in our wizard as well.

Also there is now a better way to get the 'metadata' than to parse the html form. See here:

https://github.com/spring-io/initializr/wiki/Initializr-metadata-json-format

It would be good to use this metadata as parsing the html is a pretty fragile way of doing it.