Update sbt-ghpages to 0.9.0 (#249)

Author: scala-steward

project/plugins.sbt

@@ -2,6 +2,6 @@ resolvers += "jgit-repo" at "https://download.eclipse.org/jgit/maven"
 
 addSbtPlugin("com.github.sbt" % "sbt-site"       % "1.6.0")
 addSbtPlugin("com.github.sbt" % "sbt-release"    % "1.4.0")
-addSbtPlugin("com.github.sbt" % "sbt-ghpages"    % "0.7.0")
+addSbtPlugin("com.github.sbt" % "sbt-ghpages"    % "0.9.0")
 addSbtPlugin("com.github.sbt" % "sbt-ci-release" % "1.11.2")
 addSbtPlugin("org.scalameta"  % "sbt-scalafmt"   % "2.5.5")