(RE-2195) Exclude pkg directory when globbing gem

Previously, the gem task would build the generic gem, and
then build platform specific gems. However, the second gem
would include the previously built gem.

This commit excludes the pkg directory when globbing gem files.
Merge pull request #305 from joshcooper/ticket/master/RE-2195-exclude-pkg

(RE-2195) Exclude pkg directory when globbing gem