Precompute SpringBootVersion at build time

[philwebb]

Currently org.springframework.boot.SpringBootVersion does a lot of work at runtime to get the version number. We should be able to make this a lot simpler by either putting it in a properties file or generate code at build time.

[wilkinsona]

Does the new file need to be in every jar? I wonder if it could just go in the spring-boot jar?