Compilation error on GCC7

[dukesun99]

I got something similar like json.hpp:6395:66: error: wrong number of template arguments (1, should be 2) return *lhs.m_value.array < *rhs.m_value.array; when compiling with GCC 7.4.0 on ubuntu 18.04. After searching I found this error is due to a regression of GCC7 and later, making json library fail to compile. They addressed this issue in compiling with gcc 7 -> error on bool operator < #590. Upgrading the json library to the newest version will solve this bug. But I am not sure whether this will introduce new bugs or not. At least it can compile now. I can raise a pr to fix it if you want.

Btw thanks for the code!