Release 0.3.0

[dwijnand]

• A milestone exists for the release you are about to perform. If it doesn't create one.
• The milestone due date is the day of the release.
• Travis passes against latest Scala versions (past example: bump Scala version to 2.12.1 in Travis script #164)
  https://travis-ci.org/lightbend/migration-manager/builds/382591712
• All tickets fixed in the current development cycle are associated to the current milestone.
• Bump the version in the README.md and git commit.
• Run clean. (particularly if you've recently bumped scalaVersion - see #191)
• Create an annotated, signed git tag (git tag -a -s) and push it. The name of the tag should follow the format of previous tags (not 'v' prefix). reload and verify that show version in sbt picks up the version number from the tag.
• Run ^publishSigned. You should start seeing "published mima-.. to https://oss.sonatype.org/service/local/staging/deploy/maven2/.."
• Login on Sonatype and follow this guide to release the staged MiMa artifacts.
• Login to Bintray and publish sbt plugin artifacts. (or use sbtplugin/bintrayRelease)
• Wait for artifacts to show up on Maven Central
  • sbt plugin jar and dependent jars resolvable by sbt
  • virtual directory appears: https://repo1.maven.org/maven2/com/typesafe/mima-core_2.10/0.1.14
• Close milestone
• Add release notes to tag
  https://github.com/typesafehub/migration-manager/releases/tag/0.3.0
• Hit "Publish Release" on GitHub
• Announce the release in the Announce category of Scala Users.
  https://users.scala-lang.org/t/announcing-mima-migration-manager-0-3-0/2782