build for 2.12.0-RC1

and drop 2.12.0-M1 which is obsolete

Author: SethTisue

Diff for: build.sbt

@@ -11,9 +11,9 @@ scalaVersion               := crossScalaVersions.value.head
 crossScalaVersions         := {
   val java = System.getProperty("java.version")
   if (java.startsWith("1.6.") || java.startsWith("1.7."))
-    Seq("2.11.8", "2.12.0-M1")
+    Seq("2.11.8")
   else if (java.startsWith("1.8.") || java.startsWith("1.9."))
-    Seq("2.12.0-M5")
+    Seq("2.12.0-RC1")
   else
     sys.error(s"don't know what Scala versions to build on $java")
 }