Apply some standard Ruby style conventions

Let me preface this: I normally hate whitespace commits/pull requests.
However, there are some broadly accepted Ruby style conventions that I
think are valuable to follow, if only to make it easier for other Ruby
devs to contribute. Please feel free to disregard this PR if you don't
agree :wink:

Doing a `git diff --ignore-space-change` lists only a few actual lines
of diff, from revs 9f219f5 and e8146db. The main whitespace change is
2-space soft tab indentation, which hides those two real changes since
every line has been touched.