drop support for Scala 2.11 as of 0.10.0

[SethTisue]

also upgrade Scala 2.12.4 -> 2.12.6

[SethTisue]

@retronym the motivation here is to pave the way to move to sbt 1 by dropping Java 7 support

[SethTisue]

if this is merged, at the same time we'll need to the create a new 2.11.x branch, which README.md references

[SethTisue]

the broader issue of sbt 1 support for modules is being discussed at scala/sbt-scala-module#32

here in this repo, there's no great need to be on sbt 1. so Jason, I guess this basically depends on whether you consider dropping 2.11 support to be a nice simplification that would make your life in this repo a little easier, going forward.

[retronym]

Thanks. Agree the time is right.

This will simplify #194, as no longer have to avoid using Java SAMs.

[retronym]

if this is merged, at the same time we'll need to the create a new 2.11.x branch, which README.md references

Done: https://github.com/scala/scala-async/tree/2.11.x

[SethTisue]

2.11 community build updated: scala/community-build@20a94d9