[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 checkstyle/checkstyle#3771).

git-svn-id: https://svn.apache.org/repos/asf/maven/plugins/trunk@1782677 13f79535-47bb-0310-9956-ffa450edef68

Author: Guillaume Boué

pom.xml

@@ -63,7 +63,7 @@ under the License.
     <maven.compiler.source>1.${javaVersion}</maven.compiler.source>
     <maven.compiler.target>1.${javaVersion}</maven.compiler.target>
     <mavenVersion>3.0</mavenVersion>
-    <checkstyleVersion>6.11.2</checkstyleVersion>
+    <checkstyleVersion>6.18</checkstyleVersion>
     <doxiaVersion>1.4</doxiaVersion>
     <sitePluginVersion>3.4</sitePluginVersion>
   </properties>