Use File.expand_path with require.

For some reason, even though our own tests pass with `require
puppet_x/apt_key/patch_openuri` every other module that depends on
puppetlabs-apt is now breaking in tests claiming it can't load that
module.

Somewhere along the way something is probably messing with LOADPATH
causing this to trip up. This should fix the issues for everyone.

This commit affects:
 * puppetlabs/puppetlabs-apt#229
 * puppetlabs/puppetlabs-postgresql#391