Update .travis.yml

.travis.yml

 language: python
 sudo: required
-dist: xenial
+dist: bionic
 python:
   - "3.5"
   - "3.6"
   - "3.7"
+  - "3.8"
 env:
-  - FLASK=1.1.1
+  - FLASK=1.1.2
   - FLASK=1.0.4
-  - FLASK=0.12.4
+  - FLASK=0.12.5
 install:
   - pip install Flask==$FLASK
   - pip install -r dev_requirements.txt