Change version to 1.25.0 (#3146)

[skip ci] Skipping Travis. Version number change only
diff --git a/version.go b/version.go
index 87509fc..0e71b95 100644
--- a/version.go
+++ b/version.go
@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.25.0-dev"
+const Version = "1.25.0"