Update sbt-mima-plugin to 0.6.4 (#233)

* Update sbt-mima-plugin to 0.6.4

* Update sbt-mima-plugin to 0.6.4

Author: scala-steward

project/plugins.sbt

@@ -1,5 +1,5 @@
 addSbtPlugin("com.geirsson" % "sbt-ci-release" % "1.5.2")
-addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.3")
+addSbtPlugin("com.typesafe" % "sbt-mima-plugin" % "0.6.4")
 addSbtPlugin("org.scala-js" % "sbt-scalajs" % "0.6.32")
 addSbtPlugin("org.tpolecat" % "tut-plugin" % "0.6.13")
 addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "3.8.1")