Travis-CI config for both recent(ish) gcc and clang

On account of the age of Travis-CI's build images (I hear) getting a new-ish C++ compiler going is futzy. After much futzing, the following .travis.yml file works for an autoconfiscated project:

language: generic
script: ./bootstrap && ./configure && make all && make check && make distcheck
matrix:
  include:
    - os: linux
      env: COMPILER_NAME=gcc CXX=g++-5 CC=gcc-5
      addons:
        apt:
          sources:
            - ubuntu-toolchain-r-test
          packages:
            - autotools-dev
            - g++-5
    - os: linux
      env: COMPILER_NAME=clang CXX=clang++-3.8 CC=clang-3.8
      addons:
        apt:
          sources:
            - ubuntu-toolchain-r-test
            - llvm-toolchain-precise-3.8
          packages:
            - autotools-dev
            - clang-3.8