.travis.yml-matrix

#    # works on Precise and Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - ubuntu-toolchain-r-test
#          packages:
#            - g++-5
#      env:
#         - MATRIX_EVAL="CC=gcc-5 && CXX=g++-5"
#
#    # works on Precise and Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - ubuntu-toolchain-r-test
#          packages:
#            - g++-6
#      env:
#        - MATRIX_EVAL="CC=gcc-6 && CXX=g++-6"
#
#    # works on Precise and Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - ubuntu-toolchain-r-test
#            - llvm-toolchain-precise-3.7
#          packages:
#            - clang-3.7
#      env:
#        - MATRIX_EVAL="CC=clang-3.7 && CXX=clang++-3.7"
#
#    # works on Precise and Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - ubuntu-toolchain-r-test
#            - llvm-toolchain-precise-3.8
#          packages:
#            - clang-3.8
#      env:
#        - MATRIX_EVAL="CC=clang-3.8 && CXX=clang++-3.8"
#
#    # works on Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - llvm-toolchain-trusty-3.9
#          packages:
#            - clang-3.9
#      env:
#        - MATRIX_EVAL="CC=clang-3.9 && CXX=clang++-3.9"
#
#    # works on Trusty
#    - os: linux
#      addons:
#        apt:
#          sources:
#            - llvm-toolchain-trusty-4.0
#          packages:
#            - clang-4.0
#      env:
#        - MATRIX_EVAL="CC=clang-4.0 && CXX=clang++-4.0"