MNT: Added scikit-image to conda install of travis yaml file
MNT: Apparently the --yes flag is important...

Turns out that if you don't add --yes to conda package [install]
then travis won't run because it's waiting for someone to
say 'yes, install this package i told you to install...'
Merge pull request #7 from ericdill/powder_center_corr

BLD : Added scikit-image to conda install of travis yaml file