Make pairwise distances passed to scikit nearest neighbors nonnegative

Minor bugfix release corresponding to pull request #40

@rosaxma received the error ValueError: Negative values in data passed to 'pairwise_distances'. Precomputed distance need to have non-negative values when scikit's NearestNeighbors functions were called. This fix shifts all the distances upwards so that they are all nonnegative, which appears to eliminate the error without affecting the results. I am not sure why this error wasn't encountered before - it may have to do with the particular version of scikit.