do not use the imgmath option here, doesn't seem to be supported by rtd.

Antti Haapala · Antti Haapala · commit 24dacfb6574e · 2016-11-24T23:18:42.000+02:00
diff --git a/source/conf.py b/source/conf.py
@@ -35,7 +35,7 @@
     'sphinx.ext.intersphinx',
     'sphinx.ext.todo',
     'sphinx.ext.coverage',
-    'sphinx.ext.imgmath',
+#    'sphinx.ext.imgmath',
     'sphinx.ext.ifconfig',
     'sphinx.ext.viewcode',
     'sphinx.ext.githubpages',
