Try to lock lower bound of deps to sebastian/comparator 2.0

Author: Jean85

composer.json

@@ -19,8 +19,12 @@
     },
     "require-dev": {
         "phpspec/phpspec": "^3.4 || ^4.2",
+        "phpspec/prophecy": "^1.7",
         "guzzlehttp/psr7": "^1.4"
     },
+    "conflict": {
+        "sebastian/comparator": "<2"
+    },
     "suggest": {
         "php-http/logger-plugin": "PSR-3 Logger plugin",
         "php-http/cache-plugin": "PSR-6 Cache plugin",