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