prevent circle from building gh-pages

Author: yyx990803

build/update-docs.sh

@@ -1,6 +1,7 @@
 cd docs
 rm -rf _book
 gitbook build
+cp _circle.yml _book/circle.yml
 cd _book
 git init
 git add -A

docs/_circle.yml

@@ -0,0 +1,4 @@
+general:
+  branches:
+    ignore:
+      - gh-pages