Updated installed_OS regex

Slashes in old regex were escaping the ".", making it literally interpreted.

testing on rhel7
````
[shawnw@ssgdev-rhel7 oval]$ rpm -qv redhat-release-server
redhat-release-server-7.0-1.el7.x86_64
[shawnw@ssgdev-rhel7 oval]$ ./testcheck.py installed_OS_is_rhel6.xml 
Evaluating with OVAL tempfile : /tmp/installed_OS_is_rhel6t_Hp87.xml
Writing results to : /tmp/installed_OS_is_rhel6t_Hp87.xml-results
Definition oval:scap-security-guide.testing:def:100: false
Evaluation done.
[shawnw@ssgdev-rhel7 oval]$ ./testcheck.py installed_OS_is_rhel7.xml 
Evaluating with OVAL tempfile : /tmp/installed_OS_is_rhel7Dclx9j.xml
Writing results to : /tmp/installed_OS_is_rhel7Dclx9j.xml-results
Definition oval:scap-security-guide.testing:def:110: true
Evaluation done.
````