add `-p` flag to `ghp-import` so that it pushes

ci/github_pages.sh

@@ -6,7 +6,7 @@ BOOK_DIR=book
 # Only upload the built book to github pages if it's a commit to master
 if [ "$TRAVIS_BRANCH" = master -a "$TRAVIS_PULL_REQUEST" = false ]; then
     mdbook build 
-    ghp-import $BOOK_DIR
+    ghp-import -p $BOOK_DIR
 else
     echo Skipping 'mdbook build' because this is not master or this is just a PR.
 fi