Fix Travis builds and add build for ESP8266 (#586)

* Fix Travis builds and add build for ESP8266

examples/simple_server/simple_server.ino

@@ -6,8 +6,13 @@
 //
 
 #include <Arduino.h>
+#ifdef ESP32
 #include <WiFi.h>
 #include <AsyncTCP.h>
+#elif defined(ESP8266)
+#include <ESP8266WiFi.h>
+#include <ESPAsyncTCP.h>
+#endif
 #include <ESPAsyncWebServer.h>
 
 AsyncWebServer server(80);