Change version to 1.39.0 (#4541)

[skip ci] Skipping Travis. Version number change only

Co-authored-by: Zach Reyes <[email protected]>

Author: dfawley

version.go

@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.39.0-dev"
+const Version = "1.39.0"