This commit upgrades to Spring Framework 5.0.0.RC1. It also upgrades
the Spring Boot-based samples to use Spring Boot 2.0.0.M1 which
supports Spring Framework 5. Lastly, it upgrades a number of
third-party dependencies to align with Spring Framework 5 and Spring
Boot 2.
Closes gh-386
Rather than requiring an output directory to be explcitly configured,
a default is now automatically configured based on the build tool
that's being used. When using Gradle, snippets will be generated in
build/generated-snippets. When using Maven, snippets will be
generated in target/generated-snippets.
See gh-297
