Increase minimum Dask version and remove try/except imports

[GenevieveBuckley]

We have this try/except statement for imports here:

dask-image/dask_image/ndmeasure/_utils/__init__.py

Lines 14 to 17 in bbe73c6

	try:
	from dask.array import blockwise as da_blockwise
	except ImportError:
	from dask.array import atop as da_blockwise

According to John:

Yeah this was renamed in Dask 1.1.0 ( dask/dask#4348 ). If we are comfortable making that a minimum requirement, we can drop that and just use dask.array.blockwise below

Originally posted by @jakirkham in #227 (comment)

Reply from Gen:

I suspect that changing the minimum requirement for Dask is going to involve some fiddling with the environment files we use for the CI, so I'll open an issue & that can be a separate PR.

[GenevieveBuckley]

Thank you @jakirkham