as_grey has been deprecated in favor of as_gray

[dapperfu]

Warning testing out caffe on Ubuntu 18.04 with Python 3.6 and the latest scikit-image:

lib/python3.6/site-packages/skimage/io/_io.py:49: UserWarning: `as_grey` has been deprecated in favor of `as_gray`

  warn('`as_grey` has been deprecated in favor of `as_gray`')

[Hunandaxueheying]

I met the same problem

[hyper750]

Will be helpful if anyone can review the pull request and merge to master, thanks.