try fixing build docs on RTD

Author: benbovy

doc/environment.yml

@@ -3,17 +3,16 @@ channels:
   - conda-forge
   - defaults
 dependencies:
-  - attrs
-  - python=3.5
-  - numpy=1.12
-  - pandas=0.22
-  - xarray=0.10.0
-  - ipython=6.3.1
-  - ipykernel=4.6.1
-  - matplotlib=2.0.2
+  - attrs=18.2.0
+  - python=3.6
+  - numpy=1.14.5
+  - pandas=0.23.3
+  - xarray=0.10.9
+  - ipython=6.4.0
+  - matplotlib=2.2.2
   - graphviz
   - python-graphviz
   - nbconvert=5.2.1
-  - sphinx=1.7.4
+  - sphinx=1.7.6
   - nbsphinx=0.3.3
   - pandoc=1.19.2