Update IEEE 802.15.4 Beacon Version value to 2. (#409)

diff --git a/src/core/mac/mac_frame.hpp b/src/core/mac/mac_frame.hpp
index 787c121..158cb3c 100644
--- a/src/core/mac/mac_frame.hpp
+++ b/src/core/mac/mac_frame.hpp
@@ -686,7 +686,7 @@
 
     enum
     {
-        kProtocolVersion  = 1,                      ///< Thread Protocol version.
+        kProtocolVersion  = 2,                      ///< Thread Protocol version.
         kVersionOffset    = 4,                      ///< Version field bit offset.
         kVersionMask      = 0xf << kVersionOffset,  ///< Version field mask.
         kNativeFlag       = 1 << 3,                 ///< Native Commissioner flag.