[MRG] Change the default of precompute from "auto" to False 

The Gram variant has been found to be slower, then the normal update rules. See the discussion in https://github.com/scikit-learn/scikit-learn/pull/3220 . So the default is changed from "auto" to False.
why 0.18?

I would raise the warning in 0.16 and remove in 0.17