Require PHP >= 7.1

Fix #3

composer.json

@@ -18,7 +18,12 @@
             "HAB\\Pica": "src/"
         }
     },
+    "require": {
+        "php": "^7.1"
+    },
     "require-dev": {
-        "phpunit/phpunit": "^8"
+        "phpunit/phpunit": "^8",
+        "phpstan/phpstan": "^0.11.15",
+        "phan/phan": "^2.2"
     }
 }