Change version to 1.31.1-dev (#3756)

diff --git a/version.go b/version.go
index fb878dd..a5e95d1 100644
--- a/version.go
+++ b/version.go
@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.31.0"
+const Version = "1.31.1-dev"