Backport to same bytecode level as io.zipkin.brave:brave

[codefromthecrypt]

There's no reason to have this bytecode JDK 1.8, and it can break api later to rely on it and have to move back after people are using this library. Propagation is the most low-level api, so we should be conservative even if we design interfaces to fit functional ones and/or method references.

Let's move back to source 1.6 (with retrolambda so that the IDE still feels like 1.8) and setup animal sniffer to ensure there is no accidental leaks of functions, streams classes etc.