[Releasing] Updating the mingw version for 5.4 (CI/packaging)

[Massimo Belgrano]

Why not use SEH exception?
> 
> Hi,
> 
> I'd like to suggest updating the MinGW version we build binaries for once more 
for Qt 5.4. The suggested
> toolchain would be mingw-builds i686-4.9.1-release-posix-dwarf-rt_v3-rev1 [1].
> 
> The toolchain includes:
>