Move gem_publisher require inside rake task.

gem_publisher does not support ruby 1.8.  This was causing the matrix
builds to fail in travis.  Moving this require inside the rake task
should prevent it being loaded until we actually try to publish the gem,
which in turn should allow the matrix builds to complete.
Merge pull request #22 from gds-operations/fix_gem_publisher_require

Move gem_publisher require inside rake task.