Fix erb template.

This fixes the bug introduced in commit d7be61a73bf2e7f9c39b3004500336d111085613
where the package_url erb template variable was refactored
to the top of the file. This causes that the arguments
like ${platform} is evaluated before the platform
variable is populated inside install_chef() function
Merge pull request #44 from Applifier/fix/erb-template

Fix erb template.