Actually, specify PHP version constraints for dev and non-dev separately (dev is only higher because of the dependency tree)

composer.json

@@ -8,7 +8,11 @@
     "config": {
         "vendor-dir": "administrator/components/com_patchtester/vendor"
     },
+    "require": {
+        "php": "^5.3.10|^7.0"
+    },
     "require-dev": {
+        "php": "^5.6|^7.0",
         "joomla/crowdin-sync": "dev-master",
         "squizlabs/php_codesniffer": "~1.5"
     },

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "c79698e1367cae449ff51bfa58b85cf9",
+    "content-hash": "35e372f08cd218b7040fd37d8e7dcaeb",
     "packages": [],
     "packages-dev": [
         {
@@ -1066,6 +1066,10 @@
     },
     "prefer-stable": false,
     "prefer-lowest": false,
-    "platform": [],
-    "platform-dev": []
+    "platform": {
+        "php": "^5.3.10|^7.0"
+    },
+    "platform-dev": {
+        "php": "^5.6|^7.0"
+    }
 }