Change NUTS average acceptance ratio to 0.8

Stan changed their target acceptance ratio of the
NUTS sampler recently to 0.8:
https://twitter.com/betanalpha/status/537574485302714368
because of strong theoretical considerations:
http://arxiv.org/abs/1411.6669
Merge pull request #661 from pymc-devs/nuts_accept_0.8

Change NUTS average acceptance ratio to 0.8