Specify the epub theme to fix the epub build on RTD.
diff --git a/docs/conf.py b/docs/conf.py
index fbd6d57..3486fb3 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -270,3 +270,5 @@
 
 # Example configuration for intersphinx: refer to the Python standard library.
 intersphinx_mapping = {'http://docs.python.org/': None}
+
+epub_theme = 'epub'