Remove Windows support.

With the move to tokio 1.0, it looks likely that tokio-serial will drop
support for Windows until tokio adds a Windows equivalent of `AsyncFd`.
(See berkowski/tokio-serial#33)

Let's stop running CI on Windows in response. Not removing cfg platform
gates out of the code for now in case we are able to restore Windows as
target later on.

README.md

@@ -5,7 +5,6 @@ A [tokio](https://tokio.rs)-based modbus library.
 [![Crates.io version](https://img.shields.io/crates/v/tokio-modbus.svg)](https://crates.io/crates/tokio-modbus)
 [![Docs](https://docs.rs/tokio-modbus/badge.svg)](https://docs.rs/tokio-modbus/)
 [![Build Status](https://travis-ci.org/slowtec/tokio-modbus.svg?branch=master)](https://travis-ci.org/slowtec/tokio-modbus)
-[![Build status](https://ci.appveyor.com/api/projects/status/dw7jfj7hcbmykqh0/branch/master?svg=true)](https://ci.appveyor.com/project/flosse/tokio-modbus/branch/master)
 [![Coverage Status](https://coveralls.io/repos/github/slowtec/tokio-modbus/badge.svg?branch=master)](https://coveralls.io/github/slowtec/tokio-modbus?branch=master)
 
 ## Features