Add missing dask dependency and make minimum version numbers explicit

[astrofrog]

scikit-image now has dask[array] >= 0.5 as a runtime dependency - without this, I've run into errors like:

In [1]: from pkg_resources import WorkingSet, Requirement

In [2]: w = WorkingSet()

In [3]: r = Requirement('scikit-image')

In [4]: w.resolve([r])
---------------------------------------------------------------------------
DistributionNotFound                      Traceback (most recent call last)
<ipython-input-4-5fc9dbd41de2> in <module>()
----> 1 w.resolve([r])

/home/tom/miniconda3/envs/test-stable-glue-3/lib/python3.6/site-packages/setuptools-27.2.0-py3.6.egg/pkg_resources/__init__.py in resolve(self, requirements, env, installer, replace_conflicting)
    852                     if dist is None:
    853                         requirers = required_by.get(req, None)
--> 854                         raise DistributionNotFound(req, requirers)
    855                 to_activate.append(dist)
    856             if dist not in req:

DistributionNotFound: The 'dask[array]>=0.5.0' distribution was not found and is required by scikit-image

when using setuptools to import modules that depend on scikit-image.

I've also added the minimum required versions while I was at it.

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[jni]

Thanks @astrofrog!

[jakirkham]

Sorry I didn't see this until now. The build number should have been bumped. Please see PR ( #6 ) for the fix.