Adding PHPUnit

Author: Nyholm

composer.json

@@ -20,6 +20,7 @@
         "php-http/discovery": "^1.2"
     },
     "require-dev": {
+        "phpunit/phpunit": "^6.0",
         "amphp/phpunit-util": "^1",
         "php-http/client-integration-tests": "^0.6.0",
         "php-http/message": "^1.6"
@@ -28,6 +29,7 @@
         "php-http/client-implementation": "1.0"
     },
     "scripts": {
-        "test": "vendor/bin/phpunit"
+        "test": "vendor/bin/phpunit",
+        "test-ci": "vendor/bin/phpunit --coverage-text --coverage-clover=build/coverage.xml"
     }
 }