The gitignore ** pattern is not supported by git until version 1.8.2.

CentOS 6 uses 1.7.1, Ubuntu 12.04 LTS uses 1.7.9.

https://github.com/git/git/blob/master/Documentation/RelNotes/1.8.2.txt
Merge pull request #1656 from GawainLynch/hotfix/gitignore

The gitignore ** pattern is not supported by git until version 1.8.2.