godep: switch to -copy=true

Greetings, @kr here!

I'm sending this pull request because this project appears to be using `godep save -copy=false`, and that flag is being retired. You can read more about why it's going away at the original proposal at http://goo.gl/RpYs8e. In short, godep will benefit from a more focused feature set, and `-copy=true` (the default behavior) is better in almost every way.

The procedure used to create this patch was simple, essentially:

```
godep restore
godep save $packages
```

where `$packages` are the packages (if any) listed in Godeps.

But note that this patch was generated by an automated process, so
you might want to double check that everything looks reasonable
before merging.

I hope you'll consider merging this patch—your users will thank you!

Don't hesitate to let me know if you have any questions or concerns.

Thanks!