MRG: Fixed precompute issue (again) in ElasticNet and enet_path 

The previous fix (https://github.com/scikit-learn/scikit-learn/commit/41ae851643625a2dc933fd0864a20bbd23a4e8cb) didn't change things (https://github.com/scikit-learn/scikit-learn/issues/1734) in the n_samples <= n_features case, as the value of precompute was unchanged and remained truthy.

Also, the same issue existed in enet_path, so I applied the same fix.