[RFR]: Added Origin CA Certificate endpoint (#123)

* Added Certificate endpoint

* Implement all calls in /certificates endpoint

* Added to travis more php versions

* Fix package compatibility

* Added CertificateTest

* Added test for Certificate endpoint

* Added more tests

* Updated README

Co-authored-by: haphan <thanhha.phan@myrepublic.net>

composer.json

@@ -9,7 +9,7 @@
     "ext-json": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "5.7.5",
+    "phpunit/phpunit": "^5.7",
     "phpmd/phpmd" : "@stable",
     "friendsofphp/php-cs-fixer": "^2.6"
   },