Another attempt to get book building correctly (#165)

* Delete rendered html files in docs folder

* Change local_dir in travis yml

Author: KristinaRiemer

.travis.yml

@@ -16,5 +16,5 @@ deploy:
   keep-history: true
   on: 
     branch: master
-  local_dir: docs
+  local_dir: _book
   target_branch: gh-pages