Make sure the build succeeds immediately after checkout - II

This the updated patch with the right indentation & a comment describing what checkExtensionKey (it is a filter which removes "pack-extension-key" from the opts array if the vimium.pem is missing).

This incorporates the same fix as before, for the following error I faced when I was running "cake package" immediately after checking out from git.

$ cake.coffeescript package
private key not exist
option not valid

[usage]
required opt

After the first successful package run, crxmake does create a vimium.pem for later use automatically.