prevent Facter error when packages not installed

on RHEL systems, `pkg.retrieve[pkg.property(:ensure)]` returns `:absent`
when a package is not installed; this can generate an unsightly error
when running these facts on a RHEL system where the necessary OS
packages are not installed:

    [root@gepeto ~]# facter -p osfamily
    RedHat
    [root@gepeto ~]# facter -p lsbmajdistrelease
    5
    [root@gepeto ~]# grep version /etc/puppet/modules/python/Modulefile
    version      '1.6.3'
    [root@gepeto ~]# facter -p virtualenv_version pip_version
    Could not retrieve virtualenv_version: can't convert Symbol into String
    Could not retrieve pip_version: undefined method `[]' for nil:NilClass
    Could not retrieve pip_version: can't convert Symbol into String
    pip_version =>
    virtualenv_version =>

Fixes #50
Merge pull request #60 from huit/el5_facts_error_message

prevent Facter error when packages not installed