There are three "deeper" changes included with this update:
1) The `Detach` function got removed from the `StorageActionsService` in
favor of `DetachByDropletID` (which is now used in
`resource_digitalocean_volume.go`).
2) The `Update` function got removed from `TagsService` (renaming a tag
has been deprecated in the API).
3) Every function in godo now takes a `context.Context` as first
argument, so I've changed all calls to send in a `context.Background()`.