Fix broken phpmd in travis

The phpmd/phpmd project in the version => 2.9.0 is broken with phpcq/coding-standard.
The class alias is removed from the binary, see below.
phpmd/phpmd@678a704#diff-facf4c9a9b641c017dba7c2c8aa036dd

The phpcq/coding-standard project has phpmd configuration for ExcessiveClassComplexity. This used the newer exists class of `PHP_PMD_Rule_Design_WeightedMethodCount`.
https://github.com/phpcq/coding-standard/blob/8c1c806befcacd49bfc45ed3a1321ade40896984/phpmd/ruleset.xml#L32

composer.json

@@ -53,7 +53,8 @@
         "phpunit/phpunit": "^7.5",
         "contao/manager-plugin": "^2.8",
         "menatwork/contao-multicolumnwizard-bundle": "^3.4",
-        "php-http/guzzle6-adapter": "^2.0"
+        "php-http/guzzle6-adapter": "^2.0",
+        "phpmd/phpmd": "~2.8.2"
     },
     "autoload": {
         "classmap": [