Puppet Agent fails to upgrade on Windows 2008R2

In this commit: https://github.com/puppetlabs/puppetlabs-puppet_agent/commit/95a6082dd4a6ae96e0156846ee4625489084f105 we've extracted the log function into a helper file so we can re-use it in all the scripts.

However, on Powershell v2 (shipped with Windows 2008) the $PSScriptRoot variable does not exist causing the upgrade to file.