[#2420] Fix RHEL7 detection.

Problem?
-----------

RHEL7 uses a version scheme of 7.X.xxxx and the string chopping in `paver.sh` used to obtain the major version only functions properly for strings of the form X.xxx. 

Solution?
------------

When chopping `.*` from the end of the string we should remove the longest match.

How to test?
--------------

`paver lint`
Check that the following only returns the major version in RHEL 4/5/6/7:
```
rhel_version=`cat /etc/redhat-release | sed s/.*release\ // | sed s/\ .*//`; echo ${rhel_version%%.*}
```