[FIX] update sbt-idea to version 1.6.0

I saw `No "scala-library*.jar" in Scala compiler library` error in IDEA. It seems upgrading `sbt-idea` to 1.6.0 fixed the problem.

Author: Xiangrui Meng <meng@databricks.com>

Closes #419 from mengxr/idea-plugin and squashes the following commits:

fb3c35f [Xiangrui Meng] update sbt-idea to version 1.6.0

project/plugins.sbt

@@ -8,7 +8,7 @@ addSbtPlugin("com.eed3si9n" % "sbt-assembly" % "0.10.2")
 
 addSbtPlugin("com.typesafe.sbteclipse" % "sbteclipse-plugin" % "2.2.0")
 
-addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.5.1")
+addSbtPlugin("com.github.mpeltonen" % "sbt-idea" % "1.6.0")
 
 // For Sonatype publishing
 //resolvers += Resolver.url("sbt-plugin-releases", new URL("http://scalasbt.artifactoryonline.com/scalasbt/sbt-plugin-releases/"))(Resolver.ivyStylePatterns)