Modifies regex to correctly recognize centos-7.

CentOS 7's /etc/redhat-release outputs:
CentOS Linux release 7.0.1406 (Core)

This modifies the regex to parse this format in addition to the previous
centos-6 format.
The function returns information about the osname, label, and version and uses regex groups to identify/extract them. I actually need to modify the (Linux)? part to be in the first group for this to be correct.