Use composer for some libraries (twig, phpmailer, semver)

composer.json

@@ -0,0 +1,7 @@
+{
+    "require": {
+        "phpmailer/phpmailer": "^6.1",
+        "composer/semver": "^3.2",
+        "twig/twig": "~1.42.5"
+    }
+}