Fixing qEI.grad.R to work with 1d data

[nrontsis]

When qEI.grad.R is called on 1d data an error occurs.

Minimal example (R v.3.3, DiceOptim v.2):

library('DiceOptim')
x_all <- replicate(1, rnorm(5))
y_all <- matrix(0, nrow=5, ncol=1)
m <- km(design=data.frame(x_all), response=data.frame(y_all), coef.cov=c(.1), coef.var=c(1))
x <- replicate(1, rnorm(6))
qEI.grad(x, m)

This PR addresses this issue.