Update Arduino Core to 2.4.1 // Resolve platformio#76

Author: ivankravets

platform.json

@@ -12,7 +12,7 @@
     "type": "git",
     "url": "https://github.com/platformio/platform-espressif8266.git"
   },
-  "version": "1.6.0",
+  "version": "1.7.0-beta.1",
   "packageRepositories": [
     "https://dl.bintray.com/platformio/dl-packages/manifest.json",
     "http://dl.platformio.org/packages/manifest.json",
@@ -46,7 +46,7 @@
     "framework-arduinoespressif8266": {
       "type": "framework",
       "optional": true,
-      "version": "~1.20400.0"
+      "version": "~1.20401.0"
     },
     "framework-esp8266-rtos-sdk": {
       "type": "framework",