Fixed README. More Go versions added for testing

exporter/csv.go

@@ -14,7 +14,6 @@ type CSVExporter struct {
 	FileName string
 
 	once   sync.Once
-	mut    sync.Mutex
 	writer *csv.Writer
 }
 

exporter/json.go

@@ -14,7 +14,6 @@ type JSONExporter struct {
 	EscapeHTML bool
 
 	once    sync.Once
-	mut     sync.Mutex
 	encoder *json.Encoder
 }