Update Cheffile

Yum 2.3.0 was causing issues with the sudo and uchiwa repo's referred, causing chef-librarian to go into infinite loop. Updating to 3.0.0 solves this issue.
Merge pull request #271 from sheyden/patch-1

Update yum to 3.0.0 in examples/Cheffile to avoid infinite loop in librarian-chef