Change version to 1.15.1-dev (#2302)

[skip ci] Skipping Travis. Version number change only
diff --git a/version.go b/version.go
index 8ee619b..5f04548 100644
--- a/version.go
+++ b/version.go
@@ -19,4 +19,4 @@
 package grpc
 
 // Version is the current grpc version.
-const Version = "1.15.0"
+const Version = "1.15.1-dev"