lnd_services: bump minimum required lnd version to v0.18.4-beta

With the latest lnd master versioned as v0.18.99-beta, we can now
require this commit of lndclient to require a minimum version of
v0.18.4-beta (which will ship all the RPC changes in this PR).

lnd_services.go

@@ -37,11 +37,11 @@ var (
 	// required in lnd to get all functionality implemented in lndclient.
 	// Users can provide their own, specific version if needed. If only a
 	// subset of the lndclient functionality is needed, the required build
-	// tags can be adjusted accordingly. This default will be used as a fall
-	// back version if none is specified in the configuration.
+	// tags can be adjusted accordingly. This default will be used as a
+	// fallback version if none is specified in the configuration.
 	minimalCompatibleVersion = &verrpc.Version{
 		AppMajor:  0,
-		AppMinor:  17,
+		AppMinor:  18,
 		AppPatch:  4,
 		BuildTags: DefaultBuildTags,
 	}