No way to stop travis-cargo from adding a feature flag on nightly

[Aatch]

Not a huge problem, since you can always just add a dummy feature to Cargo.toml but if you don't actually need a feature for unstable/nightly, it's a bit annoying that you can't tell travis-cargo that you don't want a flag at all.

[huonw]

You can set TRAVIS_CARGO_NIGHTY_FEATURE="" which will remove this problem. (I'm still unsure about whether I want to implement a "real" way to disable it or not.)

[Aatch]

I tried that, it treats an empty string as it not being there (I think) and uses the default.

[elinorbgr]

A workaround would be setting the variable to " ", which cargo interprets as an empty features list.

However, there are also situations where cargo simply refuses to get a --features flag.
I have a project consisting of several libs in the same git repo, that I used to test using commands like

cargo test -p foo

But after migrating to travis-cargo, cargo complains that

features cannot be modified when the main package is not being built

It would really be nice to have a way to completely disable passing the --features flag.

[huonw]

@vberger the -p problem is covered by #14 (which I'm pushing a fix for in the next few minutes).