fix JDK 6 on CI, and bump Scala and sbt and versions

[SethTisue]

modeled after @ashawley's scala/scala-xml#271

log for a green run from that PR, for reference: https://travis-ci.org/scala/scala-xml/builds/484411533

[SethTisue]

once this is green, but before I merge it, I should at least try to add openjdk11 to the Travis-CI matrix, too

[SethTisue]

LOL FML

[error] /home/travis/build/scala/scala-collection-compat/project/LatestScala213.scala:4: object file is not a member of package java.nio
[error] import java.nio.file._
[error]                 ^
[error] one error found
[error] (compile:compileIncremental) Compilation failed

[SethTisue]

I'm just going to rip out this latest-Scala-2.13 stuff, since we're so close to RC1.

(It would make a cool sbt plugin for those wanting to use Scala nightlies — but, not tackling that myself, not right now anyway.)

[SethTisue]

gah, as I get deeper into this, I'm finding that Scalafix doesn't work on Java 6, and Scalafix stuff is all through the build now, since this repo has Scalafix rules as well as the library.

let's just drop JDK 6 support instead. probably the artifacts we build will actually work on Java 6. if somebody finds they don't, they can report it and we can fix it. probably the number of people who will want to use this library on JDK 6 is small/zero.

[SethTisue]

version bumps are now #174