Handle multi-command resultsets

[pbrisbin]

Simplecov supports merging coverage from multiple test suites by writing them
at different command names in the resultset file, then merging them when
building the report.

We were getting this for free pre-1.0 because the resultsets would be merged
before being passed to the custom formatter. Post-1.0 we were (mistakenly)
assuming it'd just be one command name in the JSON file, hence we were only
reading:

SimpleCov::Result.new(results.values.fetch(0).fetch("coverage"))

discarding all but the first command-name.

This is both broken and more work than we need to do. The file has a
well-defined schema, and we can rely more on Simplcov for parsing it.

Something like:

SimpleCov::Result.from_hash(results.first)

would've been functionally equivalent, and shows more clearly the bug. What we
really want is:

results.map do |k, v|
  SimpleCov::Result.from_hash(k => v)
end

Then with an array of results, we can in-line some of Simplecov::ResultMerger
to merge them together into a single Simplecov::Result that we'll format for
upload. Unfortunately, we do have to in-line the method we need because this
object has multiple responsibilities (yay!), including caching and re-writing
files on disk, which we don't want.

Fixes #7

/cc @codeclimate/review

If someone's around and wants to merge / release that'd be great. If not, I'll
pick it up when I'm back next week.

[maxjacobson]

Nice! Going to review more closely a littler later and may ship it this week. Looks like this build is failing for reasons unrelated to your change, which should be fixed here: #164

[maxjacobson]

This LGTM, and I did a small amount of QA on a repo with only one command in the resultset. The tests look good, though. Planning to ship tomorrow.

[maxjacobson]

I like this spec description :)