Renamed all Veritas mentions to Axiom

I tried to see what's new in DM2 but I noticed that @dkubb change Veritas's name to Axiom. Looks like this was a [planned change](http://solnic.eu/2012/12/20/datamapper-2-status-and-roadmap.html) . So I update the code to use Axiom gems. 

Tests passed so I assumed all is OK

+ spec/unit: 435 examples, 0 failures, 31 pending 
+ spec/isolation: 24 examples, 0 failures
+ spec/integration: 32 examples, 0 failures, 4 pending

I'm new in DM so maybe I forgot something,