Update dependencies

Author: Yifan Wu

.travis.yml

@@ -1,7 +1,13 @@
 language: python
+
 sudo: false
+
 python:
   - "3.6"
   - "3.7"
   - "3.8"
+
+install:
+  - pip install -r requirements.txt
+  
 script: "python3 test.py"

requirements.txt

@@ -0,0 +1 @@
+yaml==5.1.2