ART: Bump runtime version to 2.1.0

Bump the version of ART to 2.1.0 to reflect recent changes like
native MultiDex support.

Change-Id: Idcff71faac932c2b30359fbe13a975fb818ecb0e
diff --git a/runtime/runtime.h b/runtime/runtime.h
index 8776a59..f839be1 100644
--- a/runtime/runtime.h
+++ b/runtime/runtime.h
@@ -231,7 +231,7 @@
   }
 
   static const char* GetVersion() {
-    return "2.0.0";
+    return "2.1.0";
   }
 
   void DisallowNewSystemWeaks() EXCLUSIVE_LOCKS_REQUIRED(Locks::mutator_lock_);