message: Change python on travis to 3.6 to avoid error
  I've tried various fixes like:
  ```
  python:
  - 3.6
  ```
  in the .travis.yml file, but none worked (see travis-ci/travis-ci/issues/8363). Keeping `pyenv global system` and simply changing the version to 3.6 worked though.
diff: @@ -8,7 +8,7 @@ env:
  - C_COMPILER=clang-5.0 CXX_COMPILER=clang++-5.0
  before_install:
  - - pyenv global system 3.5 # Workaround for travis-ci/issues/8363
  + - pyenv global system 3.6 # Workaround for travis-ci/issues/8363
  - sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y
  - wget -O - http://llvm.org/apt/llvm-snapshot.gpg.key | sudo apt-key add -
  - echo "deb http://llvm.org/apt/trusty/ llvm-toolchain-trusty-5.0 main" | sudo tee -a /etc/apt/sources.list
  @@ -22,7 +22,7 @@ before_install:
  before_script:
  - mkdir build
  - cd build
  - - ../cmake-3.8.2-Linux-x86_64/bin/cmake -DCMAKE_C_COMPILER=${C_COMPILER} -DCMAKE_CXX_COMPILER=${CXX_COMPILER} -DEOS_BUILD_EXAMPLES=on -DEOS_BUILD_UTILS=on -DEOS_GENERATE_PYTHON_BINDINGS=on -DPYTHON_EXECUTABLE=`which python3.5` ..
  + - ../cmake-3.8.2-Linux-x86_64/bin/cmake -DCMAKE_C_COMPILER=${C_COMPILER} -DCMAKE_CXX_COMPILER=${CXX_COMPILER} -DEOS_BUILD_EXAMPLES=on -DEOS_BUILD_UTILS=on -DEOS_GENERATE_PYTHON_BINDINGS=on -DPYTHON_EXECUTABLE=`which python3.6` ..
  script:
  - make VERBOSE=1
  