Merge pull request #284 from getsentry/use-phpstan-0.12

Use PHPStan shim to anticipate 0.12 release and avoid require conflicts

Author: Jean85

composer.json

@@ -34,7 +34,7 @@
         "jangregor/phpstan-prophecy": "^0.3.0",
         "monolog/monolog": "^1.11||^2.0",
         "php-http/mock-client": "^1.0",
-        "phpstan/phpstan": "^0.11",
+        "phpstan/phpstan-shim": "^0.11",
         "phpstan/phpstan-phpunit": "^0.11",
         "phpunit/phpunit": "^7.5",
         "scrutinizer/ocular": "^1.4",