Add CMake 3.17 to Travis-CI build.

.travis.yml

@@ -26,7 +26,6 @@ compiler: gcc
 
 env:
 # Python 2.7 and 3.6 are the only generations provided by pyenv on Travis-CI out-of-the-box
-  - CI_MPI=0 PY=3.5
   - CI_MPI=0 PY=3.6
   - CI_MPI=0 PY=3.7
   - CI_MPI=0 PY=3.8
@@ -35,7 +34,6 @@ addons:
   apt:
     packages:
     - doxygen
-    - g++-6
     - libblas-dev
     - libcr-dev
     - libfftw3-dev
@@ -49,6 +47,26 @@ before_install:
   - source ./ci_scripts/set_compilers
   - export CCACHE_DIR=$HOME/.ccache_gmxapi
   - source ./ci_scripts/set_python
+  # Ref https://riptutorial.com/cmake/example/4723/configure-travis-ci-with-newest-cmake
+  # first we create a directory for the CMake binaries
+  - DEPS_DIR="${TRAVIS_BUILD_DIR}/deps"
+  - mkdir ${DEPS_DIR} && cd ${DEPS_DIR}
+  # we use wget to fetch the cmake binaries
+  - travis_retry wget --no-check-certificate https://github.com/Kitware/CMake/releases/download/v3.17.1/cmake-3.17.1-Linux-x86_64.tar.gz
+  # this is optional, but useful:
+  # do a quick checksum to ensure that the archive we downloaded did not get compromised
+  - echo "23dd30da0bacf0e644d82298907b8e03edbc59c4ed40839afdeeb3b86e66bc93  cmake-3.17.1-Linux-x86_64.tar.gz" > cmake_sha256.txt
+  - sha256sum -c cmake_sha256.txt
+  # extract the binaries; the output here is quite lengthy,
+  # so we swallow it to not clutter up the travis console
+  - tar -xvf cmake-3.17.1-Linux-x86_64.tar.gz > /dev/null
+  - mv cmake-3.17.1-Linux-x86_64 cmake-install
+  # add both the top-level directory and the bin directory from the archive
+  # to the system PATH. By adding it to the front of the path we hide the
+  # preinstalled CMake with our own.
+  - PATH=${DEPS_DIR}/cmake-install:${DEPS_DIR}/cmake-install/bin:$PATH
+  # don't forget to switch back to the main build directory once you are done
+  - cd ${TRAVIS_BUILD_DIR}
 
 install:
   - export CCACHE_DIR=$HOME/.ccache_gmxapi

ci_scripts/set_compilers

@@ -1,7 +1,7 @@
 export MPICC=`which mpicc`
 export MPICXX=`which mpicxx`
-export CC=`which gcc-6`
-export CXX=`which g++-6`
+export CC=`which gcc`
+export CXX=`which g++`
 export CCACHE_COMPILERCHECK=content
 ${CC} --version
 ${CXX} --version