Fixes to load STL10 dataset

The STL10 dataset cannot be loaded because an incorrect use of parameters: y and y_labels of DenseDesignMatrix. Sanity check at _check_labels at DenseDesignMatrix was failing.
Update stl10.py

Removed spaces from line.
Merge pull request #1303 from gaa-cifasis/patch-3

Fixes to load STL10 dataset