concat: added force

Because of a potential bug in puppetlabs-concat force is needed here. See https://github.com/puppetlabs/puppetlabs-concat/commit/00a8f4e73695eb93529942b31d0580df17ee9393#diff-4050c10d82ce9debdbe92764ddf021e4R101 for the comment on that bug.
Merge pull request #49 from empi89/master

concat: added force