chore(package): update autoprefixer to version 6.4.0
chore(package): update autoprefixer to version 6.4.0

https://greenkeeper.io/
gil--/magento2-theme-blank-sass,SnowdogApps/magento2-theme-blank-sass,gil--/magento2-theme-blank-sass,SnowdogApps/magento2-theme-blank-sass