Remove go1.13 from go.mod, bump version to v0.20.1

Go 1.13 was accidently added, this backs it back out.
I don't think there's really much impact from this but I would prefer it
not be in there for this release.

discord.go

@@ -21,7 +21,7 @@ import (
 )
 
 // VERSION of DiscordGo, follows Semantic Versioning. (http://semver.org/)
-const VERSION = "0.20.0"
+const VERSION = "0.20.1"
 
 // ErrMFA will be risen by New when the user has 2FA.
 var ErrMFA = errors.New("account has 2FA enabled")

go.mod

@@ -4,5 +4,3 @@ require (
 	github.com/gorilla/websocket v1.4.0
 	golang.org/x/crypto v0.0.0-20181030102418-4d3f4d9ffa16
 )
-
-go 1.13