Upgrade to Maven Checkstyle Plugin 2.13

This release fixes an issue we encountered with parallel builds:

https://jira.codehaus.org/browse/MCHECKSTYLE-230
https://jira.codehaus.org/browse/MCHECKSTYLE-244

Release notes:

http://www.mail-archive.com/announce@maven.apache.org/msg00611.html
JCLOUDS-692 Remove the CloudSigma v1 provider.
JCLOUDS-735 Remove ninefold providers.