Fix web-php template config

This PR fixes some minor issue with the web-php template configuration:
* now using a public URL to download the bow
* updated checksum directives to be compatible with packer 0.2.0

I don't know what to do with this : https://github.com/ubermuda/packer-templates/blob/master/web-php/template.json#L20-L24
Too bad we can't add comment to the JSON file to avoid errors because packer can't stat the local file...

Hope this patch helps.

BTW, thanks for the work you did on this template!