Migrate to Java 8

[clementdenis]

With the shutdown of Java 7 App Engine standard runtime coming soon, what about upgrading the code base of Cloud Endpoints framework to Java 8?

I'm available to help :-)

[tangiel]

Yeah, I will probably work on that. Will also want to bump the Guava version, which actually involves changing the artifact to a new one, as Guava now has different artifacts for Java 7- and Java 8+. At that point I will probably bump the minor version.