Throw an error if db:migrate runs with spree_sample:load
Throw an error if db:migrate runs with spree_sample:load

This is because running these two tasks at the same time produces an error:

http://ci.spreecommerce.com/viewLog.html?buildId=2490&buildTypeId=bt28&tab=buildLog#_focus=335

lzcabrera/spree-1-3-stable,lzcabrera/spree-1-3-stable,lzcabrera/spree-1-3-stable