Update ibnn_bo tutorial to use LogEI (#2499)

Summary:
Pull Request resolved: #2499

Changing EI to LogEI in the iBNN tutorial.

Notably this anecdotally improves the performance of the Matern-GP BO substantially, even in this high-dimensional space:
{F1834660285}

Reviewed By: esantorella

Differential Revision: D61997648

fbshipit-source-id: c59ad52536c8603d123d8ca1fd3f4f9d66b87a5a

Author: SebastianAment