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.
Adds comment to clarify regex.
Merge pull request #95 from Sarsate/centos-7-config

Modifies regex to correctly recognize centos-7.