Jialei gh297 speed up gpu computation on ei grad ei

********* PEOPLE *************
Primary reviewer: @suntzu86 

Reviewers: @sc932 

********* DESCRIPTION **************
Branch Name: jialei_gh297_speed_up_gpu_computation_on_EI_gradEI
Ticket(s)/Issue(s): Closes #297 

********* TESTING DONE *************
make test
cpplint.py
same comments for this function as in the computeEI function. (add const or restrict where appropriate, document the memory size and layout, etc)