Bump symfony/security from 2.8.50 to 3.4.48

Bumps [symfony/security](https://github.com/symfony/security) from 2.8.50 to 3.4.48.
- [Release notes](https://github.com/symfony/security/releases)
- [Changelog](https://github.com/symfony/security/blob/4.4/CHANGELOG.md)
- [Commits](symfony/security@v2.8.50...v3.4.48)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

composer.json

@@ -3,7 +3,7 @@
         "php": ">=7.0.0",
         "silex/silex": "~2.1",
         "monolog/monolog": "~1.22",
-        "symfony/security": "~2.8.18",
+        "symfony/security": "~3.4.48",
         "cnam/security-jwt-service-provider": "~2.1.1",
         "doctrine/dbal": "~2.5.12",
         "egulias/email-validator": "~2.1.7",