docs: Add explicit image config for RTD

achimnol · achimnol · commit 085d94a521ad · 2018-01-03T13:09:12.000+09:00
* ref: readthedocs/readthedocs.org#3339
diff --git a/.readthedocs.yml b/.readthedocs.yml
@@ -1,4 +1,6 @@
 requirements_file: requirements-docs.txt
+build:
+  image: latest
 python:
   version: 3.6
 formats:
