Use preferred no publish setting for sbt

Seems that since 1.0 of sbt, the preferred "do not publish" setting
is `skip in publish := true`. Update the build to batch this config
and also add the missing sbt-sonatype plugin.

build.sbt

@@ -85,9 +85,7 @@ lazy val commonSettings = Seq(
 )
 
 lazy val noPublishSettings = Seq(
-  publish := {},
-  publishLocal := {},
-  publishArtifact := false
+  skip in publish := true
 )
 
 lazy val publishSettings = Seq(

project/plugins.sbt

@@ -1,6 +1,7 @@
 addSbtPlugin("com.typesafe.sbt" % "sbt-git" % "0.9.3")
 addSbtPlugin("com.jsuereth" % "sbt-pgp" % "1.1.0")
 addSbtPlugin("com.github.gseitz" % "sbt-release" % "1.0.7")
+addSbtPlugin("org.xerial.sbt" % "sbt-sonatype" % "2.2")
 
 addSbtPlugin("com.eed3si9n"      % "sbt-unidoc"  % "0.4.1")
 addSbtPlugin("org.wartremover" % "sbt-wartremover" % "2.2.1")