Remove --travis from `cargo new`

Remove --travis from `cargo new`

It's a little odd that Cargo only supports travis right now, and as #517 points
out it probably shouldn't be a top-level flag but rather a general "service
flag". Due to a lack of other services and the ease of adding your own travis
configuration, this commit removes the flag altogether for now. It can always be
added later when we've got some more impetus!

Closes #517
Closes #1115