update travis config

.travis.yml

@@ -1,6 +1,6 @@
 sudo: false
 dist:
-  - xenial
+  - bionic
 addons:
   apt:
     packages:
@@ -43,9 +43,3 @@ notifications:
   email:
     on_success: change
     on_failure: change
-  webhooks:
-    urls:
-      - https://webhooks.gitter.im/e/1aa78fbe15080b0c2e37
-    on_success: change  # options: [always|never|change] default: always
-    on_failure: always  # options: [always|never|change] default: always
-    on_start: false     # default: false