I saw a tweet from Mike Mearls that addresses this, *Cantrips are intended to use your character level*. The example give I believe was a lvl 20 high elf barbarian being able to cast firebolt at max level. The spells refer generally to character level and not spell caster level.  

The [Sage Advice Compendium][1] confirmed this when it was published. 


  [1]: https://media.wizards.com/2015/downloads/dnd/SA_Compendium_1.03.pdf