message: Travis-CI: Bump minimum gcc version to 4.9
  nlohmann-json and pybind11 (which will be added sometime)
  require a minimum gcc of 4.9.
  This bumps our minimum gcc version in Travis-CI to gcc 4.9, which
  will soon probably also be the minimum required version for building
  Avogadro2.
diff: @@ -23,7 +23,13 @@ matrix:
  - clang-format-6.0
  - compiler: gcc
  - env: TASKS="ctest gcc-4.8"
  + addons:
  + apt:
  + sources: ubuntu-toolchain-r-test
  + packages: g++-4.9
  + env:
  + - TASKS="ctest gcc-4.9"
  + - MATRIX_EVAL="CC=gcc-4.9 && CXX=g++-4.9"
  - compiler: gcc
  addons:
  