Replaced gitignore file with content as in all other barclamps

I renamed the gitignore within
https://github.com/tboerger/barclamp-nagios/commit/dd4fe83e07cac5a13dc52124ecdbd4d13d305644
wrong. Simply renamed it to .gitignore now.
Merge pull request #84 from tboerger/feature/gitignore-replace

Replaced gitignore file with content as in all other barclamps