Allow gradle builds without version numbers

Add a gradle `ResolutionStrategy` that resolves missing version numbers
using the `spring-boot-dependencies` POM.

This commit also introduces a new `spring-boot-dependency-tools` module
that provides programmatic access to the `dependencyManagement` section
of the dependencies POM.

Fixed gh-262
Update CLI to use spring-boot-dependency-tools

Update `GroovyCompiler` and `AetherGrapeEngineFactory` to use the
recently added `spring-boot-dependency-tools` in favor of loading
dependency information from a generated properties file.