CC-49: Fix the name of the pkg artifact created by the pipeline.

Currently `mac_pkg` creates the artifact with name `chef.pkg` which is not compatible with the omnitruck release logic. 

This change is tested here: http://ci.opscode.us/job/chef-client-build/1340/build_os=mac_os_x_10_7,machine_architecture=x64,role=oss-builder/

Created a package with name: `chef-11.10.0.alpha.1-246-gc861b2d-1.mac_os_x.10.7.2.pkg`

/cc: @opscode/client-eng / @sethvargo / @schisamo