use conda for rtd build

Author: delgadom

docs/environment.yml

@@ -4,7 +4,7 @@ channels:
   - defaults
 dependencies:
   - python=3.6
-  - pandas=0.20.3
+  - pandas
   - xarray
   - scipy
   - numpy
@@ -20,9 +20,4 @@ dependencies:
     - PyYAML>=4.2b1
     - pytest==3.1.3
     - pytest-runner==2.11.1
-    - pytest-cov==2.5.1
-    - py==1.4.34
     - pytest-cov==2.7.1
-    - scipy==1.3.0
-    - xarray==0.12.1
-    - toolz==0.9.0