pom.xml: updated maven-checkstyle-plugin to 2.12.1

@see https://mail-archives.apache.org/mod_mbox/maven-announce/201404.mbox/%3C2083864723.01398361442235.JavaMail.Dennis@Dennis-Dator%3E

No functional changes.
Fix warning from CheckStyle about long line.

Should be in e3eb3e8a1df0d7c5a6a8f2e4f7787556ed00afb3 commit.

No functional changes.