change dependency of bundler/gem_tasks in Rakefile

Require the whole bundle is not needed since may 2011
[Changelog](https://github.com/bundler/bundler/blob/master/CHANGELOG.md#1014-may-27-2011)
[Commit, changing dependency](https://github.com/bundler/bundler/commit/3903dfd5038aefd078bca0cba3533b155b615591)