Change version to 1.21.0 (#2828)

Can Guler · web-flow · commit 869adfc8d5a4 · 2019-05-22T10:02:58.000-07:00
[skip ci] Skipping Travis. Version number change only
diff --git a/version.go b/version.go
@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.21.0-dev"
+const Version = "1.21.0"
