Backport bug fixes

This fixes an "undefined method `fixture_path`" error caused in
`RSpec::Core::Configuration`. The issue occurs when a spec is created
before rspec-rails is loaded. The cause is rspec-core attempts to
include the `RSpec::Rails::FixtureSupport` module into the existing spec
as soon as we tell the configuration about it, but have not told the
configuration about the new related options.

This moves the including of the module until after we have told the
configuration about the dependent options.

This includes a spec which is named in a manner to attempt to ensure it
is the first spec loaded by rspec-core; which loads spec files
alphabetically. To ensure we test the issue the run script is updated to
also run this spec file standalone.

When discussing rspec/rspec-rails#1355 it was agreed that we need to
consider removing this global inclusion of the fixture support. Instead
favoring including it in the `RailsExampleGroup`. A deprecation comment
has been included to remind us later.

This removes the unnecessary warning about a preview path set to a
location other than `/spec/mailers/preview`.

This does not force `ActionMailer` to load when the Railtie runs by
removing the `defined?` check. This is necessary because the
`ActionMailer` class only loads it's settings once, after it is first
loaded. If we load the class in our Railtie any custom settings declared
in a config block in an initializer will be ignored.

An improved spec suite for the different permutations of availability of
action mailer previews is included as well.

If a view example's description contains no path elements, then do not attempt to load a helper based on the example description.

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.

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

[ci skip]

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.

[skip ci]

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

Fix #1301.

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

This reverts commit ed975be1.

Remove unnecessary code 💀

The matchers no longer attempt to use `AssertionFailedError` instead
they use `ActiveSupport::TestCase::Assertion`.

Fix #1294

Change type meta tag in templates to Hash 1.9 syntax

Update directory structure documentation [ci skip]

The website docs were not considering that request specs could also be
located inside a folder named 'integration' or 'api'.

Added a feature check that uses the Rails version to decide if we can
use 1.9 hash syntax for our generatored specs

Make `instance_double` et al work with ActiveRecord columns.

Update RailsVersions.md with recent Rails versions

[ci skip]