Merge pull request #427 from QuantEcon/fix-rtd

update to new rtd requirements spec

Author: mmcky

readthedocs.yml

@@ -1,5 +1,10 @@
-conda:
-    file: rtd-environment.yml
+# .readthedocs.yml
+
+build:
+  image: latest
+
 python:
-    version: 3.5
-    setup_py_install: true
+  version: 3.6
+  setup_py_install: true
+
+requirements_file: pip-requirements.txt