Upgrade to Checkstyle 2.12.1

Checkstyle 2.12 changelog:
http://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11127&version=19723

Checkstyle 2.12.1 changelog:
http://jira.codehaus.org/secure/ReleaseNote.jspa?projectId=11127&version=20236

Checkstyle 5.7 changelog:
http://checkstyle.sourceforge.net/releasenotes.html