Pick mixlib-shellout 1.3.0.rc.0 to prepare for 10.30.0.rc.0

Normally we do tricks in omnibus-chef to get the RC versions of the gems we would like to include in the chef release candidates. However since we don't have a different project for mixlib-shellout in the omnibus-chef or omnibus-software here we are temporarily changing the gem dependency of the chef gem. 

Even though this is not the ideal solution, workarounds via omnibus-chef would cause multiple versions of mixlib-shellout or multiple versions of dependencies to be installed.