Revert "Use latest cmake for the main build, too, with better Python support

This reverts commit d4a5198.

cmake 3.14 can't find py3 installation when "python" points to Python 3 while cmake 3.9 can

Author: native-api

travis_config.sh

@@ -111,10 +111,6 @@ function pre_build {
         brew_install_and_cache_within_time_limit ffmpeg_opencv || { [ $? -gt 1 ] && return 2 || return 0; }
     else
         brew install ffmpeg_opencv
-        #cmake is a build dependency of ffmpeg so a bottle should be present in cache
-        if (brew outdated | grep -qxF cmake) || ! (brew list | grep -qxF cmake); then
-            _brew_install_and_cache cmake 1 0
-        fi
     fi
 
     if [ -n "$CACHE_STAGE" ]; then