Add support for go-module

go.mod

@@ -0,0 +1 @@
+module github.com/gorilla/websocket

go.sum

@@ -0,0 +1,2 @@
+github.com/gorilla/websocket v1.4.0 h1:WDFjx/TMzVgy9VdMMQi2K2Emtwi2QcUQsztZ/zLaH/Q=
+github.com/gorilla/websocket v1.4.0/go.mod h1:E7qHFY5m1UJ88s3WnNqhKjPHQ0heANvMoAMk2YaljkQ=