Update Rakefile

* Upgraded PE from 3.0.1 to 3.1.0
* Upgraded CentOS from 6.3 to 6.4
Making the iso_url for CentOS more generic.
Fix the Debian iso_url and actually fix the CentOS iso_url.
Merge pull request #154 from klynton/bugfix-centosupgrade

Update PE and CentOS iso_url in Rakefile.