How do I install OS X Yosemite on an unsupported MacBook?
You should use following to make a bootable Yosemite USB.

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ OS\ X\ Yosemite.app —nointeraction


Just replace the Untitled with name of the USB drive