Update composer.json

composer.json

@@ -9,7 +9,7 @@
 		"issues": "https://forge.typo3.org/projects/extension-tinymce"
 	},
 	"require": {
-		"typo3/cms-core": "*"
+		"typo3/cms-core": "7.6"
 	},
 	"replace": {
 		"tinymce": "self.version",