ulimit simple implementation

Should solve it for CentOS (tested).

Neither /etc/pam.d/common-session, nor /etc/default/riak
are modified, but they might have to be; see
https://github.com/basho/riak-chef-cookbook/commit/41661c227e1a985a52ee87678a91588d05fbe372
and
http://docs.basho.com/riak/latest/cookbooks/Open-Files-Limit/.
Merge pull request #17 from basho/feature/ulimit-centos

ulimit simple implementation

This relates to issue #13 - @Iristyle - could you check which of the options - `/etc/default/riak` or `/etc/pam.d/common-session` should be used and whether that differs between versions of debian/ubuntu?