remove docs before building

Co-Authored-By: Jang Rush <[email protected]>

Author: juvenn

Makefile

@@ -7,7 +7,8 @@ release:
 	make doc
 
 doc:
-	vendor/bin/apigen generate --source src --destination docs
+	@rm -r docs
+	@php5.6 vendor/bin/apigen generate --source src --destination docs
 
 test_engine:
 	php -S ${LEANCLOUD_APP_HOST}:${LEANCLOUD_APP_PORT} test/engine/index.php