Added support for older versions to be backwards compatible

Author: Willem Oostendorp

.travis.yml

@@ -1,6 +1,7 @@
 language: php
 
 php:
+  - 7.0
   - 7.1
 before_install:
   - sudo apt-get update -q

composer.json

@@ -5,8 +5,8 @@
     "license": "MIT",
     "require": {
         "php": ">=5.5.9",
-        "symfony/workflow": "^4.0",
-        "symfony/process": "^4.0",
+        "symfony/workflow": "^3.3 | ^4.0",
+        "symfony/process": "^3.3 | ^4.0",
         "symfony/event-dispatcher": "^4.0",
         "illuminate/console": "5.3.* | 5.4.* | 5.5.* | 5.6.*",
         "illuminate/support": "5.3.* | 5.4.* | 5.5.* | 5.6.*"
@@ -36,6 +36,6 @@
     },
     "require-dev": {
         "mockery/mockery": "^0.9.8",
-        "phpunit/phpunit": "^6.0|~7.0"
+        "phpunit/phpunit": "^6.0 | ~7.0"
     }
 }