[TST] Unpin dask

[DPeterK]

Explore how much breaks if we unpin dask to use the latest master.

cc @corinnebosley @pp-mo

Closes #3147

[DPeterK]

Unpinning dask caused three test failures in both Python 2.7 and 3.6:

======================================================================
FAIL: test_dim0_lazy (iris.tests.unit.cube.test_Cube.Test_collapsed__lazy)
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Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.2.0a0-py3.6.egg/iris/tests/unit/cube/test_Cube.py", line 288, in test_dim0_lazy
    self.assertTrue(cube_collapsed.has_lazy_data())
AssertionError: False is not true

======================================================================
FAIL: test_dim1_lazy (iris.tests.unit.cube.test_Cube.Test_collapsed__lazy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.2.0a0-py3.6.egg/iris/tests/unit/cube/test_Cube.py", line 294, in test_dim1_lazy
    self.assertTrue(cube_collapsed.has_lazy_data())
AssertionError: False is not true

======================================================================
FAIL: test_multidims (iris.tests.unit.cube.test_Cube.Test_collapsed__lazy)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.2.0a0-py3.6.egg/iris/tests/unit/cube/test_Cube.py", line 301, in test_multidims
    self.assertTrue(cube_collapsed.has_lazy_data())
AssertionError: False is not true

----------------------------------------------------------------------
Ran 3869 tests in 220.910s
FAILED (SKIP=115, failures=3)

See https://travis-ci.org/SciTools/iris/jobs/436602113#L5603-L5628.

[DPeterK]

Looks like the test failures reported here are at most only incidentally related to dask and more to do with an oversight in Cube.collapsed. I've corrected that, so now eagerly awaiting further knock-ons from so doing.

[DPeterK]

@pp-mo @corinnebosley looks like latest dask is working 👍

[corinnebosley]

@dkillick Wow this is exciting! I really want to push the button.

[corinnebosley]

Boom! That's so satisfying.

Thanks @dkillick, good job!