Update dependencies in RTD environment (#168)

* Update dependencies in RTD environment

ReadTheDocs builds recently started failing. This seems to be due to a
`pip` option being passed that the version of `pip` we use here doesn't
understand. In an attempt to fix this issue, try modernizing our
dependencies here.

https://readthedocs.org/projects/dask-image/builds/12159489/

* Bump `pip` and `wheel` to latest

Apparently `pip` version `20.2`+ has the newer resolver, which RTD tries
to use. So bump to that version so that builds don't fail due to this
flag being passed.

environment_doc.yml

@@ -4,12 +4,12 @@ channels:
   - conda-forge
 
 dependencies:
-  - pip==18.0
-  - wheel==0.31.1
-  - sphinx==1.7.5
-  - dask-sphinx-theme==1.1.0
-  - dask==0.16.1
-  - numpy==1.11.3
+  - pip==20.2.4
+  - wheel==0.35.1
+  - sphinx==3.2.1
+  - dask-sphinx-theme==1.3.5
+  - dask==2.8.1
+  - numpy==1.15.4
   - pims==0.4.1
   - slicerator==0.9.8
   - pip: