added an upper bound to the php version constraint (#84)

* added an upper bound to the php version constraint

* trigger new travis build

* trigger build

Author: staabm

composer.json

@@ -19,7 +19,7 @@
     }
   ],
   "require": {
-    "php": ">=5.6",
+    "php": "^5.6 || ^7.0",
     "mdanter/ecc": "^0.4.0",
     "lib-openssl": "*",
     "spomky-labs/base64url": "^1.0",