Replace version pin on proj4 to permit proj 4 v5.

[pp-mo]

Simply replaces #3353, which was over-complicated by a different issue

Relates to (but no longer closes) #3154

There is a chance we'll have to exclude proj4 >= 6.0 (newly out I think).
But I think cartopy dependencies should sort that out for us.
Here goes ...

[pp-mo]

UPDATE
Tests pass in Python2, where the proj4 version is 5.2.0, and fail in Python3 where proj4 is 6.1.1

Reviewing progress on cartopy, we have a problem.
Although they fixed an initial problem whereby cartopy would not install with Proj4 v6, they did not get the tests working again, that is still outstanding
There too, tests pass in Python2 + proj4 v5, but not Python3 with proj4 v6.

But worse ... the above discussion implies that outputs have changed with the new proj4 : it means "real" changes in the behaviour.

So it seems it is not appropriate to pin proj4 < 6 in cartopy, as I had hoped, which would have made for a quick fix for Iris' sake.
Instead we will need to update our tests.
And in the case of cartographical calculations, we also have numerical failures to sort out, not just graphics, e.g. ...

Traceback (most recent call last):
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.3.0.dev0-py3.6.egg/iris/tests/unit/analysis/cartography/test_gridcell_angles.py", line 182, in test_bounds_array_args
    self.standard_small_cube_results, atol=0.1)
  File "/home/travis/miniconda/envs/test-environment/lib/python3.6/site-packages/scitools_iris-2.3.0.dev0-py3.6.egg/iris/tests/__init__.py", line 694, in assertArrayAllClose
    raise AssertionError(msg)

AssertionError: 
ARRAY CHECK FAILED "assertArrayAllClose" :
  with shapes=(5, 6) (5, 6), atol=0.1, rtol=1e-07
  worst at element (0, 0) :  a=-165.42600810043507  b=-11.505944742986964
  absolute error ~154, equivalent to rtol ~-1.338e+01

And lots, lots more : 45 failures, of which ~10 are graphics tests.
😞

[pp-mo]

@lbdreyer in view of the above, and a desire to fix this for iris 2.3, do you think it is after all appropriate to pin proj4 to <6 for Iris alone ?

[pp-mo]

pin proj4 to <6 for Iris alone ?

Now done.
Ping @lbdreyer : are you in accord with this ?

Meanwhile I update #3154. This will no longer close that one.

[lbdreyer]

pin proj4 to <6 for Iris alone ?

Now done.
Ping @lbdreyer : are you in accord with this ?

I am in accord with this approach. We really do, at some point, need to remove the pin, but there are higher priority items that we need to get started on, so I don't want to just yet sink too much time into fixing the tests to work with the latest proj4.
When we do unpin proj4, at the same time we should also get the cartopy tests passing(!) to give us assurance cartopy works with proj4 6.