Update Travis to Ubuntu "Bionic" (#14664)

.travis.yml

-dist: xenial
-sudo: require
+dist: bionic
+sudo: false
 
 language: python
 python:
 notifications:
   email: false
 
-# Cache PlatformIO packages using Travis CI container-based infrastructure
-sudo: false
-cache:
-  pip: true
-  directories:
-  - "~/.platformio"
-
 env:
   - TEST_PLATFORM="megaatmega2560"
   - TEST_PLATFORM="DUE"
   - TEST_PLATFORM="esp32"
   - TEST_PLATFORM="alfawise_U20"
 
-addons:
-  apt:
-    sources:
-      - ubuntu-toolchain-r-test
-    packages:
-      - g++-7
-
 before_install:
-  - sudo update-alternatives --install /usr/bin/g++ g++ /usr/bin/g++-7 90
   #
   # Fetch the tag information for the current branch
   - git fetch origin --tags