Revert "Use std namespace for snprintf."

This reverts commit 1c58876.

std::snprintf() is only available in C++11, which is not provided by
all compilers. Since the C library snprintf() can easily be used as a
replacement on Linux systems, this patch changes jsoncpp to use the C
library snprintf() instead of C++11 std::snprintf(), fixing the build error
below:

    src/lib_json/json_writer.cpp:33:18: error: 'snprintf' is not a member of 'std'

See #231, #224, and #218.

Author: baruchsiach

src/lib_json/json_writer.cpp

@@ -29,8 +29,6 @@
 
 #if defined(_MSC_VER) && _MSC_VER < 1500 // VC++ 8.0 and below
 #define snprintf _snprintf
-#else
-#define snprintf std::snprintf
 #endif
 
 #if defined(_MSC_VER) && _MSC_VER >= 1400 // VC++ 8.0