Merge pull request #6 from precariouspanther/compat-phpunit5

Add support for projects using older versions of PHPUnit.

Author: precariouspanther

composer.json

@@ -22,7 +22,7 @@
     "container-interop/container-interop": "^1.1"
   },
   "require-dev": {
-    "phpunit/phpunit": "^6.0"
+    "phpunit/phpunit": "^6.0 || ^5.6"
   },
   "autoload": {
     "psr-4": {