Update version to 1.6.0-dev

Branching 1.5 for studio 3.5

Change-Id: I1899bd125358d61d41077a28056b9506970d07cb
diff --git a/src/main/java/com/android/tools/r8/Version.java b/src/main/java/com/android/tools/r8/Version.java
index e625c28..34f8f13 100644
--- a/src/main/java/com/android/tools/r8/Version.java
+++ b/src/main/java/com/android/tools/r8/Version.java
@@ -11,7 +11,7 @@
 
   // This field is accessed from release scripts using simple pattern matching.
   // Therefore, changing this field could break our release scripts.
-  public static final String LABEL = "1.5.23-dev";
+  public static final String LABEL = "1.6.0-dev";
 
   private Version() {
   }