> I did some quick tests and couldn't find any significant difference in speed or disk usage using --depth,
Probably doesn't matter for most cases, but it seems wasteful to download the full history of something if we're not using it.

> I normally test gems / plugins with the latest of everything and then peg if something breaks - since the users env will vary 
The normal setup I've seen tends to peg e.g. ruby and gems ruby to specific versions (possibly with a build matrix to test multiple version). Always building the latest in Travis seems okay as long as we know which revision gets used, but I think it would be better to peg to the latest released version. With pegging a specific version, you don't get surprised by it and then have to suddenly try to figure out what went wrong.