Update Spring Boot dependencies
Add mavenCentral to Spring Boot template build.

The Spring Boot template wasn't working for new projects because the build
couldn't find the spring-core-4.0.2 dependency. This was fixed by enabling
`mavenCentral()` in the `buildscript`. I also removed some tab indents since
the rest of the build file uses spaces.
Bump Spring Boot template version to 0.2.