Fix sigil symbol deprecation warning in v0.12.5+

As of v0.12.5, using % for sigils [is deprecated][1]; the ~ character should be used instead. This change fixes up various tests and example files so that they won't throw the deprecation warning.

Thanks for the [great project](http://exercism.io/) and making Elixir a part of it!

[1]: https://github.com/elixir-lang/elixir/releases/tag/v0.12.5