add compatibility for symfony process library 2 or 3

Author: Shawn McCool

composer.json

@@ -16,7 +16,7 @@
   "require": {
     "backup-manager/backup-manager": "^1.0",
     "php": ">=5.5.0",
-    "symfony/process": "^2.0",
+    "symfony/process": "^2.0||^3.0",
     "illuminate/support": "^4.0||^5.0",
     "illuminate/container": "^4.0||^5.0",
     "illuminate/console": "^4.0||^5.0"