add fmt check to ci [not ready to merge]

[soenkehahn]

cargo fmt -- --check currently fails on master. I think we should either

• ensure on travis that cargo fmt -- --check passes or
• decide that this project is formatted manually and rustfmt isn't used.

I would prefer the first option.

[soenkehahn]

I'm a bit baffled why this passes on travis (for stable). It doesn't on my machine with the same rustfmt --version.