This *might* fix the last gcc-6 error.

See open-source-parsers#411 (comment)

I was unable to produce a warning in Clang, so I am not certain. But based on a [SO answer](http://stackoverflow.com/questions/25480059/gcc-conversion-warning-when-assigning-to-a-bitfield), I think I've fixed the following:
```
/tmp/jsoncpp/src/lib_json/json_value.cpp: In copy constructor 'Json::Value::CZString::CZString(const Json::Value::CZString&)':
/tmp/jsoncpp/src/lib_json/json_value.cpp:235:18: error: conversion to 'unsigned char:2' from 'unsigned int' may alter its value [-Werror=conversion]
   storage_.policy_ = (other.cstr_
                      ~~~~~~~~~~~~
                  ? (static_cast<DuplicationPolicy>(other.storage_.policy_) == noDuplication
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                      ? noDuplication : duplicate)
                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                  : static_cast<DuplicationPolicy>(other.storage_.policy_));
                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: cdunn2001

src/lib_json/json_value.cpp

@@ -231,7 +231,7 @@ Value::CZString::CZString(const CZString& other)
     : cstr_(other.storage_.policy_ != noDuplication && other.cstr_ != 0
                 ? duplicateStringValue(other.cstr_, other.storage_.length_)
                 : other.cstr_) {
-  storage_.policy_ = (other.cstr_
+  storage_.policy_ = static_cast<unsigned>(other.cstr_
                  ? (static_cast<DuplicationPolicy>(other.storage_.policy_) == noDuplication
                      ? noDuplication : duplicate)
                  : static_cast<DuplicationPolicy>(other.storage_.policy_));