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
Add begin/rescue/end to setcode block

This silences these warnings:

Could not retrieve pip_version: undefined method `[]' for nil:NilClass
Could not retrieve pip_version: can't convert Symbol into String
Add begin/rescue/end to setcode block

This silences these warnings:

Could not retrieve pip_version: undefined method `[]' for nil:NilClass
Could not retrieve pip_version: can't convert Symbol into String
Merge pull request #62 from genome-vendor/master

Silence warnings when programs are not installed
Release 1.6.4
adds external pkgname variable
Merge pull request #65 from marcinzaremba/fix-pip-name

Same package for mutiple virtualenvs
Release 1.6.5
Merge branch 'master' of github.com:stankevich/puppet-python

* 'master' of github.com:stankevich/puppet-python:
  adds external pkgname variable
Release 1.6.6
Document newly required pkgname option for pip.
add support for explicit version handling with ensure parameter for pip packages
Don't try to reinstall pip packages on every Puppet run #59; Even if ensure latest, do not install if already latest
Merge pull request #71 from tehgeekmeister/patch-1

Document newly required pkgname option for pip.
Merge pull request #72 from CpuID/master

add support for explicit version handling with ensure parameter for pip packages
Merge pull request #73 from giavac/master

Don't try to reinstall pip packages on every Puppet run #59
Release 1.7.0
Add pypy support
Merge pull request #75 from quizlet/master

Add pypy support