[BUGFIX] Remove the roave/security-advisories dependency (#60)

1. This dependency should only be a dev dependency, not a production
   dependency, so that is kicks in when we update packages.
2. This package is intended to be used only for projects and distributions,
   not for libraries (like this package).

Author: Oliver Klee

composer.json

@@ -31,9 +31,7 @@
     "require": {
         "php": "~7.0.0 || ~7.1.0 || ~7.2.0",
 
-        "phplist/core": "4.0.x-dev",
-
-        "roave/security-advisories": "dev-master"
+        "phplist/core": "4.0.x-dev"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.5.6",