Unbreak GCC 4.6 compatibility on this branch.

I f'ed up in 469a18556dc6034c02ccb695452c16936cea8f73 and added noexcept
workaround also for GCC 4.6, which supports it flawlessly. Now I wasted
an hour trying to figure out why Magnum works with master and not with
compatibility branch on 4.6.