Fix a rapidjson compile error in mac.

Summary:
This diff fixes the following compilation error in mac.
  ./third-party/rapidjson/reader.h:422:31: error: comparison of constant 256 with expression of type 'Ch' (aka 'char') is always true
        [-Werror,-Wtautological-constant-out-of-range-compare]
                                  if ((sizeof(Ch) == 1 || e < 256) && escape[(unsigned char)e])
                                                          ~ ^ ~~~
  1 error generated.

Test Plan: make db_test

Reviewers: haobo, sdong, igor, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19245