Build: Update sbt and plug-ins

Author: tindzk

project/build.properties

@@ -1 +1 @@
-sbt.version=1.0.4
+sbt.version=1.2.8

project/plugins.sbt

@@ -1,6 +1,6 @@
 logLevel := Level.Warn
 
-addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject"      % "0.5.0")
-addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "0.5.0")
-addSbtPlugin("org.scala-js"       % "sbt-scalajs"                   % "0.6.23")
+addSbtPlugin("org.portable-scala" % "sbt-scalajs-crossproject"      % "0.6.0")
+addSbtPlugin("org.portable-scala" % "sbt-scala-native-crossproject" % "0.6.0")
+addSbtPlugin("org.scala-js"       % "sbt-scalajs"                   % "0.6.26")
 addSbtPlugin("org.scala-native"   % "sbt-scala-native"              % "0.3.7")