Add ProcessorInterface to package processors

Close #47

Author: hedii

src/Processors/NullStringProcessor.php

@@ -3,8 +3,9 @@
 namespace Hedii\LaravelGelfLogger\Processors;
 
 use Monolog\LogRecord;
+use Monolog\Processor\ProcessorInterface;
 
-class NullStringProcessor
+class NullStringProcessor implements ProcessorInterface
 {
     /**
      * Transform a "NULL" string record into a null value.

src/Processors/RenameIdFieldProcessor.php

@@ -3,8 +3,9 @@
 namespace Hedii\LaravelGelfLogger\Processors;
 
 use Monolog\LogRecord;
+use Monolog\Processor\ProcessorInterface;
 
-class RenameIdFieldProcessor
+class RenameIdFieldProcessor implements ProcessorInterface
 {
     /**
      * Rename "id" field  to "_id" (additional field 'id' is not allowed).