Revert "lib/json.hpp: remove a compiler warning on clang < 4.0"

Revert 717eea9e. Actually this produced more warnings for all gcc versions
than we had for some versions of clang.

algo/lib/json.hpp

@@ -1831,10 +1831,7 @@ class lexer
         for (auto range = ranges.begin(); range != ranges.end(); ++range)
         {
             get();
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunsequenced"
             if (JSON_LIKELY(*range <= current and current <= *(++range)))
-#pragma clang diagnostic pop
             {
                 add(current);
             }