(RE-477) Allow any vendor_gems task access to package:vendor_gems

For razor we will be invoking bundler from within rake from within jruby to
fetch the dependencies of the Gemfile. This will be accomplished by using an
auxiliary vendor_gems task called package:jruby_vendor_gems. In order for that
task to invoke the package:vendor_gems task, it must be defined even if the
pre_tar_task isn't equal to package:vendor_gems. This commit updates the task
to be defined if any pre_tar_task is defined.
Merge pull request #236 from haus/ticket/master/re_477_define_vendor_gems_task_for_all_pretar_tasks

(RE-477) Allow any vendor_gems task access to package:vendor_gems