Correct provisioner variable in 'unpackage' action

The PEBuild::Action::Unpackage class checks both :global_config variables
and :vm specific variables to determine its operation. Without this
commit, the :vm variable referred to for 'filename' is incorrect and
referes to the :vm 'version' variable instead.

This commit corrects the reference to the :vm 'filename' configuration
variable.
Merge pull request #7 from trlinkin/action_typo

Correct provisioner variable in 'unpackage' action