do not fail without sudoversion fact on modern os's

[LadyNamedLaura]

The main problem here is that any module depending on puppet-sudo has to manually specify the sudoversion fact in its rspec tests because otherwise it will fail.
since versioncmp fails hard when you pass it undef an alternative would be something like:

if (!$::sudoversion or versioncmp($::sudoversion, '1.7.2p1') < 0)

this would still print the warning, but not fail the manifest compilation.

[saz]

I'm happy for any PR resolving this issue.

[saz]

If this issue still exists, feel free to send in a pull request. Closing this now.