[MCHECKSTYLE-322] Update plugin to use 6.19 checkstyle (last version that is based on jdk7)
[MCHECKSTYLE-322] Update plugin to use 6.19 checkstyle (last version that is based on jdk7)

Updating Checkstyle to 2.18, and not 2.19 because of a regression (see issue https://github.com/checkstyle/checkstyle/issues/3771).

git-svn-id: 6038db50b076e48c7926ed71fd94f8e91be2fbc9@1782677 13f79535-47bb-0310-9956-ffa450edef68

hgschmie/apache-maven-plugins,hgschmie/apache-maven-plugins,mcculls/maven-plugins,rkorpachyov/maven-plugins,apache/maven-plugins,hazendaz/maven-plugins,apache/maven-plugins,hazendaz/maven-plugins,rkorpachyov/maven-plugins,hgschmie/apache-maven-plugins,mcculls/maven-plugins,rkorpachyov/maven-plugins,hazendaz/maven-plugins,hazendaz/maven-plugins,hgschmie/apache-maven-plugins,apache/maven-plugins,mcculls/maven-plugins,rkorpachyov/maven-plugins,apache/maven-plugins,apache/maven-plugins,hazendaz/maven-plugins,mcculls/maven-plugins,rkorpachyov/maven-plugins