Release MiMa 0.5.0

[dwijnand]

• Check Travis CI passes against the latest Scala versions.
• Create a new milestone for the release you are about to perform, if one doesn't exist.
• Create a new milestone for the release after this one.
• Draft a new release for the release, by comparing changes.
• Bump the version in the README.md and git commit -m 'Release 0.x.y
• Run clean in sbt, particularly if you've recently bumped scalaVersion.
• Run git tag -a -s 0.x.y -m 'Version 0.x.y and git push --follow-tags. The name of the tag should NOT have a 'v' prefix. Run reload and show version in sbt to verify the version.
• Run ^publishSigned in sbt. You should start seeing "published mima-.. to https://oss.sonatype.org/service/local/staging/deploy/maven2/.."
• Find and close your stating repository. (See Sonatype's Releasing the Deployment guide.)
• Test the release by adding resolvers ++= Seq("Sonatype OSS Staging" at "https://oss.sonatype.org/content/repositories/staging", Resolver.bintrayRepo("typesafe", "sbt-plugins")) to a project
• Find and release your stating repository.
• Run sbtplugin/bintrayRelease in sbt (or use the Web UI to publish the sbt plugin artifacts.
• Close the milestone
• Wait for the artifacts to show up on Maven Central, either by:
  • successfully resolving the sbt plugin jar and dependent jars with sbt
  • successfully resolving the virtual directory: https://repo1.maven.org/maven2/com/typesafe/mima-core_2.12/0.5.0/ (note the trailing slash)
• Hit "Publish Release" on GitHub.

You are done!