Wrong MSRV in README

[YaLTeR]

A minimal Travis setup could look like this (requires Rust 1.24.0 or greater):

However, the rustfmt component is only available starting from 1.31.0. Anything below gives:

└─ rustup toolchain add 1.30.0 -c rustfmt
info: syncing channel updates for '1.30.0-x86_64-unknown-linux-gnu'
info: latest update on 2018-10-25, rust version 1.30.0 (da5f414c2 2018-10-24)
error: component 'rustfmt' for target 'x86_64-unknown-linux-gnu' is unavailable for download for channel 1.30.0

[calebcartwright]

The docs originally mentioned the rustfmt-preview component which was subsequently corrected (as that component stopped being shipped a long time ago), but obviously the version wasn't updated accordingly.

Thanks for the report @YaLTeR!

For anyone interested in fixing this, the incorrect line can be found here:
https://github.com/rust-lang/rustfmt#checking-style-on-a-ci-server

[chansuke]

I'd like to fix this.