On Go ≥ 1.13 the os package has os.ConfigDir
(https://golang.org/pkg/os/#UserConfigDir) which returns the default
root directory to use for user-specific configuration data. Use this in
internal.ConfigDir if available, while still making sure that any
directory specified in GOPS_CONFIG_DIR takes precedence.
internal object packages are no longer required
given gops depends on rsc.io/goversion/version now.
Also running dep prune to remove the packages
gops doesn't depend from the vendor directory.
Follow up of
112843115d (commitcomment-23936637)
Solves the problem of us having to manually watch
upstream and pull in bug fixes and updates, if our
internal copy goes stale.
Fixes#22.
