Remove JsonCPP from authors, no longer used.

AUTHORS.txt

@@ -16,9 +16,6 @@ ZeroTier One includes or links with the following third party software:
  * LZ4 compression algorithm by Yann Collet (BSD license)
    http://code.google.com/p/lz4/
 
- * JsonCPP by Baptiste Lepilleur (public domain)
-   http://jsoncpp.sourceforge.net
-
  * http-parser, a simple C http parser library (MIT license)
    https://github.com/joyent/http-parser