Pip package provider does not handle pip executable paths with spaces

The package pip provider has some suggestion of Windows support. However, it uses "execpipe" to invoke commands, which puts a shell redirection of "2>&1" into the final command which is not compatible with Windows.

On a Windows system with Python installed when the "pip_version" method is called it results in an error: "Cannot collect packages for Puppet::Type::Package::ProviderPip3 provider; undefined method `[]' for nil:NilClass"

Reworking this in terms of execute allows the pip provider to work on Windows as well.