Must skip warning tests if version of six is < 1.7.
Merge branch 'issue255' into devel
Finesses the check of skimage.io

If on Anaconda and python3 and the scikit image version < 0.11, then
don't bother.  Otherwise, go back to prior try/except code for existance
of skimage.io and availability of freeimage backend, which fixes the
issue on Linux Mint.
Merge branch 'issue256' into devel