Adjust SDK plugin prop: tools 21 requires ADT 21.

This will make ADT check that it's at least ADT 21
when used with tools 21.

Change-Id: I32b109b2fe81829f300db130cf474048f55e3e37
diff --git a/files/plugin.prop b/files/plugin.prop
index 6c58561..c85f504 100644
--- a/files/plugin.prop
+++ b/files/plugin.prop
@@ -1,3 +1,3 @@
 # begin plugin.prop
-plugin.version=20.0.0
+plugin.version=21.0.0
 # end plugin.prop
\ No newline at end of file