[ripple/rippled] Simplify strHex: (#2668)

Like I mentioned on your commit, we could clean up the member function detection with standard library utilities like `std::experimental::is_detected` or even `boost::hana::is_valid`, but both would require GCC 6 as the min compiler. Based on looking at https://github.com/ripple/rippled/blob/develop/Jenkinsfile#L157, it looks like we use Clang 5.0.1, but the system version of GCC (I think, since a full path isn’t specified, so it’ll find the one in system PATH) which caught me a bit by surprise. Do you mind sharing what the min GCC version supported is atm? I can’t see the Jenkins build logs unfortunately.