(packaging) Add vendor_gems pre_tar_task to dashboard

The puppet-dashboard build will need to execute the vendor_gems pre-tar
task. This commit adds this to the build_defaults.yaml file.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>
Merge pull request #227 from MosesMendoza/packaging/rails3/add_vendor_gems_task

(packaging) Add vendor_gems pre_tar_task to dashboard