Remove redundant includes in README [ci skip]

The ActsAsTree module is already included in ActiveRecord::Base.
This fixes #19.
+ Adding first performance test.

Someone [linked to a stackoverflow][so] post that mentioned that another library
that "has much better performance for read operations" than ours. This got me
looking at the performance of ActsAsTree tonight. I'm adding the first (in what
I hope is a series) performance test to the library.

Starting with the obviously-common case of deeply nested relationships, I
assumed the read performance was linear. Within and error threshold of 0.95 the
reads _are_ linear. I don't have equivalent tests for the other library, but
at least we have a jumping off point to ensure ActsAsTree performance can be
tested, validated and improved.

[so]: http://stackoverflow.com/questions/1319809/recursive-model-in-rails#comment1153380_1319859
Ensure minitest is a development dependency.
Adjusting test threshold for Travis.

There seems to be an issue in CI that stops this test from running quickly... if
it finishes at all. Reducing the threshold allows lower performance numbers in
the CI environment until we can figure out the slowdown.
Fully move the tests under MiniTest.

Still wrestling with the benchmark output on travis, I can't replicate the
failure to return from benchmark calls locally. I've been meaning to switch over
to MiniTest proper for a while now, this might kill two birds with one stone.
Upgrading to Minitest 5.2

Might as well go full-tilt with the Minitest conversion. Using the latest
version seems to work, just had to change a few classes.
Fixing descendants performance issue.

Part of the delay on Travis is a result of the memo object we're passing into
`each_with_object`. When we coerce the children call to an array rather than an
Arel collection things speed up nicely.
Revert "Upgrading to Minitest 5.2"

This reverts commit 06ff6df6a3777f2088629d855b82df8303208c73.

Rails 4 has a dependency on MiniTest 4.x... so getting to 5.2 is more pain than
it's worth at the moment. Reverting the upgrade until it's possible to do so
without annoyance.