use conda for rtd build

Author: delgadom

.readthedocs.yml

@@ -2,7 +2,6 @@ version: 2
 
 python:
    version: 3.7
-   install:
-      - requirements: requirements.txt
-      - requirements: requirements_dev.txt
-      - requirements: requirements_conda.txt
+
+conda:
+    environment: environment.yml

docs/environment.yml

@@ -0,0 +1,28 @@
+name: rtd
+channels:
+  - conda-forge
+  - defaults
+dependencies:
+  - python=3.6
+  - pandas=0.20.3
+  - xarray
+  - scipy
+  - numpy
+  - toolz
+  - pip:
+    - datafs==0.7.1
+    - bumpversion==0.5.3
+    - wheel==0.29.0
+    - flake8==3.3.0
+    - tox==2.7.0
+    - coverage==4.4.1
+    - Sphinx==1.6.3
+    - 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