Accept both phpunit 6.0/7.0 to allow composer to require latest Orchestra/TestBench version

composer.json

@@ -18,7 +18,7 @@
         "mongodb/mongodb": "^1.0.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^6.0",
+        "phpunit/phpunit": "^6.0|^7.0",
         "orchestra/testbench": "^3.1",
         "mockery/mockery": "^1.0",
         "satooshi/php-coveralls": "^2.0",