[go] Refactor raindrops to use string concatenation

Per @aprescott's [suggestion][1].

[1] :http://exercism.io//submissions/13787de337163c97c1fb7441
[go] Refactor raindrops conversion to use struct for lookup

I saw a couple of other entries use this strategy, and I like it more
than the sequential if-statements.

Submission: http://exercism.io/submissions/f8486da8a3abbb90921a82b6