[elixir] Add rna-transcription exercise
[elixir] All RNA transcription tests trivially passing

They're not *really* passing since I have hardcoded everything
[elixir] Refactor RNA conversion to be less trivial

Submission: http://exercism.io/submissions/eb63541509b95c83ff06a9c0
[elixir] Simplify DNA.to_rna implementation

Forget the private functions w/ pattern matching; just go with a simple
Enum.map.

Submission: http://exercism.io/submissions/dba800c6f738f3e43dad934c