Bandaid for protobuf 4761 (#1176)

* Temporary workaround for protobuf extension issue

protocolbuffers/protobuf#4761

* Bump gax to 0.36

* Configure comparators for the unit test

* Revert back to normal TestCase

* Install protobuf extension in the PHP 7.2 test runner

* Revert to TestCase

composer.json

@@ -18,7 +18,7 @@
         }
     },
     "require": {
-        "google/gax": "^0.35"
+        "google/gax": "^0.36"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8|^5.0",