Updated CMake in Travis configuration for C++17 support.

.travis.yml

@@ -25,7 +25,7 @@ matrix:
             - g++-6
 script:
   - if [[ "${TRAVIS_OS_NAME}" == "linux" ]]; then
-      CMAKE_URL="http://www.cmake.org/files/v3.7/cmake-3.7.0-Linux-x86_64.tar.gz";
+      CMAKE_URL="http://www.cmake.org/files/v3.9/cmake-3.9.4-Linux-x86_64.tar.gz";
       mkdir cmake-bin && wget --quiet --no-check-certificate -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake-bin;
       export PATH=${PWD}/cmake-bin/bin:${PATH};
     fi