Add formula for exercism binary

Exercism is a project that allows people to work on practice problems in a number of languages, and get feedback on their solutions by other members of the site.

@deepak originally added a pull request for this in Homebrew/homebrew#24469, but was asked to submit to the `homebrew-binary` project instead.

I submitted a pull request here (#38) a couple of months ago, which I have now closed, since that version is out of date.

@oschrenk submitted a pull request (#74) a week ago, but a new version of the binary was released a couple days ago.