Roll back to Scala.js 1.5.1

Pins back to our previously released version.  We can bump later if that comes back into fashion, but we can't unbump.

Author: rossabaker

project/plugins.sbt

@@ -3,5 +3,5 @@ addSbtPlugin("com.codecommit" % "sbt-spiewak-sonatype" % "0.22.1")
 addSbtPlugin("org.scalameta" % "sbt-mdoc" % "2.2.24")
 addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.4.3")
 addSbtPlugin("pl.project13.scala" % "sbt-jmh" % "0.4.3")
-addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.7.1")
+addSbtPlugin("org.scala-js" % "sbt-scalajs" % "1.5.1")
 addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject" % "1.1.0")