Reenable clang-3.9 build on travis

[mayeut]

Seems clang is back online & now available through apt add-on
If the PR passes test, this will be confirmed.

[AraHaan]

Nice PR, I taken some of the configuration from this PR to try to help with my build matrix on travis on my project. Unfortunately it fails with clang 3.9 on not only linux but also on OSX, and then also fails on OSX on g++-6. Would be nice if someone helps me figure out why so that way build would work because it uses C++ 14's <experimental/filesystem> header for std::experimental::filesystem::exists.

my .travis.yml file as follows:

language: cpp
matrix:
  include:
    - compiler: gcc
      addons:
        apt:
          sources:
            - ubuntu-toolchain-r-test
          packages:
            - g++-6
      env: COMPILER=g++-6
    - os: osx
      compiler: gcc
      addons:
        apt:
          packages:
            - g++-6
      env: COMPILER=g++-6
    - os: linux
      compiler: clang-3.9
      env: COMPILER=clang++-3.9
      addons:
        apt:
          sources:
            - llvm-toolchain-precise
            - ubuntu-toolchain-r-test
          packages:
            - clang-3.9
    - os: osx
      compiler: clang-3.9
      env: COMPILER=clang++-3.9
      addons:
        apt:
          sources:
            - llvm-toolchain-precise
          packages:
            - clang-3.9
sudo: required
dist: trusty
script:
  - $COMPILER -O3 -std=c++14 bin2c.cpp -lstdc++fs -o "bin2c"
  - chmod u+x ./bin2c
  - chmod u+x ./tests/VP.jpg
  - ./bin2c /custvar "IMAGE_VP" /nohead /infile "./tests/VP.jpg" /outfile "./tests/VP.hpp" /basefolder "./tests/"
  - $COMPILER -O3 -std=c++14 ./tests/VP.cpp -o ./tests/VP
env:
  global:
   - secure: "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"

before_install:
      - uname -a
      - if [[ "`uname`" == "Linux" ]]; then echo -n | openssl s_client -connect scan.coverity.com:443 | sed -ne '/-BEGIN CERTIFICATE-/,/-END CERTIFICATE-/p' | sudo tee -a /etc/ssl/certs/ca-; fi
      - if [[ "`uname`" == "darwin" ]]; then echo -n | softwareupdate --install $COMPILER; fi

addons:
  coverity_scan:
    project:
      name: "AraHaan/bin2c"
      version: 0.14.0
      description: "bin2c Conversion Tool."
    notification_email: seandhunt_7@yahoo.com
    build_command_prepend: " -O3 -std=c++14 bin2c.cpp -lstdc++fs -o bin2c"
    build_command: $COMPILER
    branch_pattern: coverity_scan

And the build from travis: https://travis-ci.org/AraHaan/bin2c/builds/188333959