depend on java6-runtime-headless

Thanks to @ebourg: depend on `java6-runtime-headless`. Also dropped `openjdk-7-jre-headless` as it's superfluous.

Author: bkanuka

project/Unix.scala

@@ -87,7 +87,7 @@ object Unix {
 
     // Debian Specific
     name in Debian    := "scala",
-    debianPackageDependencies += "openjdk-7-jre-headless | java-runtime-headless (>= 1.6)",
+    debianPackageDependencies += "java6-runtime-headless",
     // debianPackageDependencies += "libjansi-java",
 
     linuxPackageMappings in Debian += (packageMapping(