Try to update cmake on Mac travis.

travis_cmake_mac.sh

@@ -1,5 +1,9 @@
 #!/bin/bash
-brew reinstall --HEAD cmake
+brew uninstall cmake --force
+wget https://cmake.org/files/v3.12/cmake-3.12.0-rc2-Darwin-x86_64.dmg
+hdiutil convert -quiet cmake-3.12.0-rc2-Darwin-x86_64.dmg -format UDTO -o bar
+hdiutil attach -quiet -nobrowse -noverify -noautoopen -mountpoint right_here bar.cdr
+sudo installer -package right_here/cmake-3.12.0-rc2-Darwin-x86_64/cmake-3.12.0-rc2-Darwin-x86_64.pkg -target /
 sysctl -n hw.physicalcpu
 which cmake
 cmake --version