(packaging) Move rake task loading after variable determination

Previously, we would load classifier's rake tasks prior to setting the
"@version" variable. Because in the tasks we determine the value of JAR_FILE_V
using @version, and this has not been set yet, any reference to JAR_FILE_V is
incorrect (e.g. in the uber_jar task). This commit updates the Rakefile to load
the tasks after the version variable is set.

Signed-off-by: Moses Mendoza <moses@puppetlabs.com>