Update Travis CI to Ubuntu 18.04

clang-format below version 8 wants a different formatting and would
error on the already existing codebase.

.appveyor.yml

@@ -1,5 +1,8 @@
 image: Visual Studio 2017
 
+# Disable automatic build of Visual Studio files
+build: off
+
 environment:
   matrix:
   - QT_LOC: '"C:\Qt\5.12\msvc2017_64\bin\"'

.travis.yml

@@ -1,5 +1,5 @@
 language: cpp
-dist: xenial
+dist: bionic
 
 branches:
   except:
@@ -11,36 +11,12 @@ branches:
 matrix:
   include:
   - os: linux
-    addons:
-      apt:
-        sources:
-        - ubuntu-toolchain-r-test
-        packages:
-        - g++-7
     env:
     - QMAKE_SPEC=linux-g++
     - QT_SELECT="opt-qt512"
     compiler: gcc
 
   - os: linux
-    addons:
-      apt:
-        sources:
-        - ubuntu-toolchain-r-test
-        packages:
-        - g++-7
-    env:
-    - QMAKE_SPEC=linux-clang
-    - QT_SELECT="opt-qt512"
-    compiler: clang
-
-  - os: linux
-    addons:
-      apt:
-        sources:
-        - ubuntu-toolchain-r-test
-        packages:
-        - g++-7
     env:
     - USE_TIDY=TRUE
     - QT_SELECT="opt-qt512"
@@ -51,18 +27,15 @@ before_install:
   - chmod +x ./build_scripts/linux/format.sh
   - chmod +x ./build_scripts/linux/run-clang-tidy.sh
   - chmod +x ./build_scripts/linux/verify_formatting.sh
-  - wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-  - sudo add-apt-repository 'deb http://apt.llvm.org/xenial/ llvm-toolchain-xenial-8 main'
   - sudo apt update -qq
   - sudo apt install clang-format-8
   - clang-format-8 --version
   - python ./build_scripts/run-clang-format.py ./src -r --color always --clang-format-executable clang-format-8
 
 install:
-  - sudo add-apt-repository ppa:kubuntu-ppa/backports -y
-  - sudo add-apt-repository ppa:beineri/opt-qt-5.12.2-xenial -y
+  - sudo add-apt-repository ppa:beineri/opt-qt-5.12.7-bionic -y
   - sudo apt-get update -qq
-  - sudo apt-get install build-essential libgl1-mesa-dev
+  - sudo apt-get install gcc clang build-essential libgl1-mesa-dev
   - sudo apt-get install qt512-meta-minimal qt512multimedia qt512declarative qt512quickcontrols2  qt512tools  qt512base
   - sudo apt-get install libx11-dev libxt-dev libxtst-dev libpulse-dev
   - sudo apt-get install bear clang-tidy