Never default to a qualifier when none of them are set.

By default if not qualifier were set by the build process the version
subcommand would default to [current version]+[alpha1].

libbeat/version/helper.go

@@ -31,7 +31,7 @@ func GetDefaultVersion() string {
 var (
 	buildTime = "unknown"
 	commit    = "unknown"
-	qualifier = "alpha1"
+	qualifier = ""
 )
 
 // BuildTime exposes the compile-time build time information.