how to use the actool to create *.cer

MBP:bin l$ ./actool Images.xcassets --compile build --platform iOS --minimum-deployment-target 8.0

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE plist PUBLIC "-/

<plist version="1.0">

<dict>

<key>com.apple.actool.errors</key>

<array>

<dict>

<key>description</key>

<string>Unknown platform: "iOS".</string>

</dict>

</array>

</dict>

</plist>