Minor fix for the ESP8266 compiler

[rpseng]

Hi, this is a really mice library!

After some work I got it working within the ESP8266 wifi module together with a max3232 chip. This is less than USD 5 to get a RTU Modbus hardware by wifi!

The only problem is that the esp compiler gets confused with your ModbusMaster::send function. The fix is very simple, just adjust the cast:
void ModbusMaster::send(uint8_t data)
{
send((word)data);
}

We also discussed this issue here:
esp8266/Arduino#360

All the best.

[aneek231]

If this library works with ESP8266? I get this during compilation: This library only supports boards with an AVR or SAM processor. Please open an issue at https://github.com/4-20ma/ModbusMaster/issues and indicate which processor/platform you're using."

[4-20ma]

As of #29, you should be able to compile against the ESP8266.