Added tbachert/spi to config.allow-plugins of composer.json.

Author: AlexSkrypnyk

Diff for: composer.json

@@ -55,7 +55,8 @@
             "ergebnis/composer-normalize": true,
             "oomphinc/composer-installers-extender": true,
             "php-http/discovery": true,
-            "phpstan/extension-installer": true
+            "phpstan/extension-installer": true,
+            "tbachert/spi": true
         },
         "discard-changes": true,
         "sort-packages": true