Try to update cmake on Mac travis.

.travis.yml

@@ -13,6 +13,7 @@ matrix:
 before_install:
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo add-apt-repository ppa:ubuntu-toolchain-r/test -y && sudo apt-get update && sudo apt-get install -y g++-7; fi
   - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then chmod +x travis_cmake_linux.sh && ./travis_cmake_linux.sh; fi
+  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then chmod +x travis_cmake_mac.sh && ./travis_cmake_mac.sh; fi
   - git submodule update --init --recursive
 
 script:

travis_cmake_mac.sh

@@ -0,0 +1,6 @@
+#!/bin/bash
+wget https://cmake.org/files/v3.12/cmake-3.12.0-rc2-Darwin-x86_64.dmg
+sudo hdiutil attach cmake-3.12.0-rc2-Darwin-x86_64.dmg
+sudo installer -package /Volumes/cmake-3.12.0-rc2-Darwin-x86_64/cmake-3.12.0-rc2-Darwin-x86_64.pkg -target /
+sudo hdiutil detach /Volumes/cmake-3.12.0-rc2-Darwin-x86_64
+cmake --version