Use tagged source code from repo

http://docs.platformio.org/en/latest/librarymanager/config.html#version

library.json

@@ -6,6 +6,7 @@
     "type": "git",
     "url": "https://github.com/bblanchon/ArduinoJson.git"
   },
+  "version": "5.0.7",
   "authors": {
     "name": "Benoit Blanchon",
     "url": "http://blog.benoitblanchon.fr"