arduino json 6 async web server migration (#491)

* arduino json 6 async web server migration

* Update .travis.yml

fix travis error

* Revert "Update .travis.yml"

This reverts commit 37d1bf0e71.

* gitignore

* test fix travis

* fix travis

* fix proposed in #487

* fix travis

* fix travis

* fix travis

* fix travis

* ARDUINOJSON_5_COMPATIBILITY

* replaced to #if ARDUINOJSON_VERSION_MAJOR == 5

* fix

* added AsyncWebServer::end()

* added AsyncWebServer::end()

* Update WebServer.cpp

fix

* fix typo

* fix typo

library.json

@@ -12,7 +12,7 @@
     "type": "git",
     "url": "https://github.com/me-no-dev/ESPAsyncWebServer.git"
   },
-  "version": "1.2.0",
+  "version": "1.2.1",
   "license": "LGPL-3.0",
   "frameworks": "arduino",
   "platforms": ["espressif8266", "espressif32"],