Publish 2.0.0-M4 (main motivation: to release for Scala Native 0.4)

[SethTisue]

• merge PRs
• choose version number: 2.0.0-M4
  • there's been some small code changes since M3, so let's call this M4 rather than backpublish
• create the release and write the notes
  • https://github.com/scala/scala-xml/releases/tag/v2.0.0-M4
• wait for release build
  • https://travis-ci.com/github/scala/scala-xml/builds/215436529
• release the staging repos
  • https://oss.sonatype.org/#stagingRepositories
• artifacts on Maven Central
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-M3/2.0.0-M4/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_3.0.0-M2/2.0.0-M4/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.12/2.0.0-M4/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_2.13/2.0.0-M4/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_native0.4_2.12/2.0.0-M4/
  • https://repo1.maven.org/maven2/org/scala-lang/modules/scala-xml_native0.4_2.13/2.0.0-M4/
  • (not a complete list)
• [ ] post-release, bump version in README
  • not needed

[SethTisue]

Note that I did not actually try the Scala Native artifacts to see if they work.