roll 2.1.2 release

[SethTisue]

primarily to ship Scala Native support, as requested at #239 (comment)

but also some other fixes will get in: #227, #238

note to self: submit a post-release version bump PR (build.sbt, readme.md)

[SethTisue]

• tag, release notes: https://github.com/scala/scala-collection-compat/releases/tag/v2.1.2
• release job on Travis-CI: https://travis-ci.org/scala/scala-collection-compat/builds/568179335

[SethTisue]

the 2.13 JVM job ended up sending files to two different staging repos, one of which then also got mixed with the sjs-0.6/2.11 artifacts. neither would close. but dropping those two repos and re-running those two subjobs fixed it.

staging repos closed, released

now we watch (to pick an arbitrary directory, of the various ones) https://repo1.maven.org/maven2/org/scala-lang/modules/scala-collection-compat_2.13/2.1.2/

[SethTisue]

@ekrich @gabro I hope the Scala Native artifacts are all right?

[SethTisue]

note to self: submit a post-release version bump PR (build.sbt, readme.md)

#242