update Linux on Travis CI to Ubuntu 18.04.2 LTS (Bionic Beaver)

http://releases.ubuntu.com/18.04/

Author: shinnn

.travis.yml

@@ -4,7 +4,7 @@ node_js: node
 cache: npm
 jobs:
   include:
-  - dist: xenial
+  - dist: bionic
     script: node . node . test.js
   - dist: xenial
     script: CI="" npx c8 node . node -p "'Test for non-CI environments'" && npx c8 report --reporter=text-lcov > coverage.lcov && bash <(curl -s https://codecov.io/bash) -X gcov -f coverage.lcov