cargo fmt --all requires nightly

[ndelvalle]

➜ thresh git:(master) cargo fmt --all
Warning: can't set indent_style = Block, unstable features are only available in nightly channel.
Warning: can't set wrap_comments = true, unstable features are only available in nightly channel.
Warning: can't set fn_single_line = true, unstable features are only available in nightly channel.
Warning: can't set brace_style = SameLineWhere, unstable features are only available in nightly channel.
Warning: can't set control_brace_style = AlwaysSameLine, unstable features are only available in nightly channel.

I think we should update the implementation to avoid unstable features.