Fix Money::Currency.register exception when table is not initialized

[sunny]

Closes #1172

[yukideluxe]

Ouch! I actually think this is happening since #1143 because the @table is not loaded until a currency is initialized 😳 😳 😳 😳 😳 😳 😳

I would not be against releasing a new version! What do you think, @sunny!?

[sunny]

Good call about #1143, it makes sense and a git bisect confirms it. Let’s go for a v7.0.2 🚀