Get rid of conversion warning in gcc-4.8 

Get rid of the following warning:
conversion to ‘int’ from ‘long unsigned int’ may alter its value [-Werror=conversion]

This change enables FMT_FALLLTHROUGH also for GCC-Compilers in the versions 7 and 8.

* Remove conversion compiler warning

When compiling with g++8, I get the following two errors:
include/fmt/format-inl.h:400:29: error: conversion from ‘int’ to ‘char’ may change value [-Werror=conversion]
       buffer[size++] = zero + static_cast<char>(digit);
                        ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~
include/fmt/format-inl.h:416:28: error: conversion from ‘int’ to ‘char’ may change value [-Werror=conversion]
       buffer[size++] = '0' + digit;
                        ~~~~^~~~~~~

With this change, the errors are gone.

This reverts commit 2a952dd0.