This reverts commit ed975be.

Fix #1301.

This fixes an issue where Rail's autoload does not find it in some
environments.

overrides RSpec::Matchers::BuiltIn::BaseMatcher#description to
return the description "be routable" instead of "be routable matcher"

[skip ci]

The previous constraint would only go as up as `4.2.0`, thus making `rspec-rails`
incompatible with any of the patch releases of the rails 4.2.x series.

[ci skip]

Backport pull requests #1303, #1305, #1310, #1318

For some reason the test-unit gem loads it's autorunner by default.
Though the test-unit included with Ruby does not. The original code for
loading test-unit pre-dates widespread usage of the test-unit gem.

We took care of disabling the autorunner for Ruby 2.2+ since the
test-unit gem is now required for Rails 3.2. For older Ruby versions we
attempt to only load the `test/unit/assertions` module. When using the
`test-unit` gem however, Rails will auto require it for us, which loads
the main test-unit code, thus enabling the autorunner.