check if we need MINIMUM_VARIANCE_EI for monte_carlo and qEI

[denizokt]

TODO on line 716 of moe/optimal_learning/python/python_version/expected_improvement.py

[suntzu86]

This is less about whether the underflow adjustment is needed (it is) but whether adding it to EVERY entry of variance is necessary. I think you only need to modify the diagonal. Two action items:

1. do we need to modify every entry or just the diagonal?
2. is the current MINIMUM_VARIANCE_EI value sufficient for q>1 case?