Pin versions in RTD Conda environment file

When conda env is solved on readthedocs.org, it gets killed for
excessive memory consumption, so pin the versions to avoid this.
Subsequent updates will likely need version updates for new packages.

docs/rtd_env.yml

@@ -5,19 +5,19 @@ channels:
 dependencies:
   - python=3.7
   - basemap>=1.1.0
-  - dask
-  - matplotlib
-  - netcdf4
-  - numpy
-  - numpydoc
-  - pandas
-  - pip
-  - psutil
-  - scipy
+  - dask=2.1.0
+  - matplotlib=3.1.0
+  - netcdf4=1.5.3
+  - numpy=1.16.4
+  - numpydoc=0.9.1
+  - pandas=0.24.2
+  - pip=19.1.1
+  - psutil=5.6.3
+  - scipy=1.3.0
   - seaborn=0.9.0
-  - setuptools
-  - sphinx
-  - xarray
-  - yaml
+  - setuptools=41.0.1
+  - sphinx=2.1.2
+  - xarray=0.14.1
+  - yaml=0.1.7
   - pip:
     - sphinx-rtd-theme