build: bump master to version v0.18.99-beta

As is customary when preparing for the next major (or minor) release, we
bump the version to .99 to allow us to set the minimum required version
to something like v0.18.4-beta in lndclient and it would still accept
the master branch (even if that target version hasn't been
released/tagged yet).

build/version.go

@@ -43,7 +43,7 @@ const (
 	AppMinor uint = 18
 
 	// AppPatch defines the application patch for this binary.
-	AppPatch uint = 00
+	AppPatch uint = 99
 
 	// AppPreRelease MUST only contain characters from semanticAlphabet per
 	// the semantic versioning spec.