Fix fallback if atomic can't be required.

https://github.com/headius/thread_safe/commit/33114e71ad538745800f49f445effff2e60146ac removed the atomic dependency, but it still is a development dependency of the gem so in development `require 'atomic'` never fails, which means we never have to rescue `LoadError`. In a real Rails App where we updated thread_safe just now this failed obviously.