Fix issue #18368417: 5.0 -> 5.1.

Change-Id: Icd7137fe8b1bda1c5aaf170aa7b19a58832144c8
diff --git a/core/version_defaults.mk b/core/version_defaults.mk
index 1acc320..d87a032 100644
--- a/core/version_defaults.mk
+++ b/core/version_defaults.mk
@@ -41,7 +41,7 @@
   # which is the version that we reveal to the end user.
   # Update this value when the platform version changes (rather
   # than overriding it somewhere else).  Can be an arbitrary string.
-  PLATFORM_VERSION := LollipopMR1
+  PLATFORM_VERSION := 5.1
 endif
 
 ifeq "" "$(PLATFORM_SDK_VERSION)"