Bump gregwar/captcha from 1.1.4 to 1.1.8

Bumps [gregwar/captcha](https://github.com/Gregwar/Captcha) from 1.1.4 to 1.1.8.
- [Release notes](https://github.com/Gregwar/Captcha/releases)
- [Commits](https://github.com/Gregwar/Captcha/compare/v1.1.4...v1.1.8)

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

composer.json

@@ -34,7 +34,7 @@
         "swiftmailer/swiftmailer" : "5.4.8",
         "symfony/console" : "3.4.2",
         "symfony/event-dispatcher" : "3.4.2",
-        "gregwar/captcha": "1.1.4"
+        "gregwar/captcha": "1.1.8"
     },
     "autoload" : {
         "classmap" : ["app/"],