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 in the original proposal at http://goo.gl/RpYs8e. In short, `-copy=true` (the default behavior) provides a better experience for the end user, and godep will benefit from a more focused feature set.

It's not hard to switch. But I figured, to make it as easy as possible I should just do it for you. :smile:

The procedure used to create this patch was, essentially,

```
godep restore
godep save github.com/travis-ci/artifacts github.com/travis-ci/artifacts/artifact github.com/travis-ci/artifacts/client github.com/travis-ci/artifacts/env github.com/travis-ci/artifacts/logging github.com/travis-ci/artifacts/path github.com/travis-ci/artifacts/upload
```

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

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

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

:sparkling_heart: Thanks! :sparkling_heart: