message: Update Travis build to Ubuntu 20.04
  Also:
  switch to using GTK+ 3
  add libwxgtk-webview3.0-gtk3-dev dependency, now required
diff: sudo: required
  -dist: trusty
  +dist: focal
  language: cpp
  @@ -8,12 +8,7 @@ compiler: gcc
  # dependencies for building Poedit on old Ubuntu:
  addons:
  apt:
  - sources:
  - - ubuntu-toolchain-r-test
  - - sourceline: 'ppa:vslavik/poedit'
  packages:
  - - gcc-6
  - - g++-6
  - libexpat1-dev
  - libdb++-dev
  - libboost-dev
  @@ -23,26 +18,25 @@ addons:
  - libboost-iostreams-dev
  - liblucene++-dev
  - libicu-dev
  - - libwxgtk3.0-dev
  - - libgtk2.0-dev
  - - libgtkspell-dev
  + - libwxgtk3.0-gtk3-dev
  + - libwxgtk-webview3.0-gtk3-dev
  + - libgtk-3-dev
  + - libgtkspell3-3-dev
  - libcld2-dev
  - libcpprest-dev
  - libsecret-1-dev
  -env:
  - - REAL_CC='ccache gcc-6' REAL_CXX='ccache g++-6'
  -
  before_install:
  - # work around Travis being difficult, see https://github.com/travis-ci/travis-ci/issues/6633
  - - export CC="$REAL_CC" CXX="$REAL_CXX"
  # git submodules needed for Linux:
  - git submodule update --init deps/json deps/pugixml
  script:
  - scripts/travis-check-translations.sh
  # don't build directly, create a tarball first and try building from that
  - - ./bootstrap && ./configure && make dist && make distcheck
  + - ./bootstrap
  + - ./configure
  + - make dist
  + - make distcheck
  after_failure:
  - test -f Makefile || cat config.log
  