Produce valid lockfile for 7.1.27

7.1.27 is what Travis uses in its 7.1 build, which we use in the Code
  quality step.

Author: greg0ire

composer.json

@@ -15,6 +15,11 @@
             "homepage": "http://ocramius.github.com/"
         }
     ],
+    "config": {
+        "platform": {
+            "php": "7.1.27"
+        }
+    },
     "require": {
         "php": "^7.1"
     },