Try to update cmake on Mac travis.

travis_cmake_linux.sh

@@ -1,2 +1,5 @@
 #!/bin/bash
-ls /Applications
+sudo rm -rf /usr/local/cmake-3.9.2 && sudo rm -rf /usr/local/cmake
+wget https://cmake.org/files/v3.12/cmake-3.12.0-rc2-Linux-x86_64.sh
+chmod +x cmake-3.12.0-rc2-Linux-x86_64.sh
+sudo ./cmake-3.12.0-rc2-Linux-x86_64.sh --skip-license --exclude-subdir --prefix=/usr/local

travis_cmake_mac.sh

@@ -1,6 +1,2 @@
 #!/bin/bash
-wget https://cmake.org/files/v3.12/cmake-3.12.0-rc2-Darwin-x86_64.dmg
+ls /Applications
-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