Bugfixes in gaussian_process subpackage (continued)

This is a continuation of the inactive PR #2632, which fixed two issues in the gaussian_process package:
 * Optimum theta over multiple random-starts was not determined correctly
 * gp_diabetes_dataset.py crashed because it tried to access gp.theta rather than gp.theta_

Credit for these fixes goes to  @dubourg. The fixes have already been reviewed by @agramfort, @jaquesgrobler, and @GaelVaroquaux. The only thing missing was a regression test, which I add in this PR (sorry for a new PR, but the old one is apparently inactive and I can not work on it). Furthermore, I removed a call to scipy.rand in GaussianProcess which resulted in inconsistent results for the same random_state.

I hope that this is sufficient for getting this PR merged soon.