Update version for release 1.2.20-dev.

R=sgjesse@google.com

Change-Id: I7197b6207a510982986eeaf76e9e6eb10037ea22
diff --git a/src/main/java/com/android/tools/r8/Version.java b/src/main/java/com/android/tools/r8/Version.java
index 24b2cc9..0ba40b3 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 = "1.2.19-dev";
+  public static final String LABEL = "1.2.20-dev";
 
   private Version() {
   }