Bump to Rails 4.2.0.rc release line.

Locking `arel` is no longer necessary as the RC1 release has the proper
fixed arel call. However, it requires `rails-dom-testing` 1.0.5+ which
fails for us on Travis CI (linux). This allows the Rails 4.2.0 envs to
fail without killing the build, until `rails-dom-testing` is fixed.

See https://github.com/rails/rails-dom-testing/issues/28
Properly ignore 4.2 on Ruby 1.9.2 & 1.8.7
Use i18n gem < 0.7.0 under Ruby 1.9.3

i18n 0.7.0 no longer supports Ruby < 1.9.3.

https://github.com/svenfuchs/i18n/blob/v0.7.0/i18n.gemspec#L21
Updated travis build scripts (from rspec-dev)
Fix failing scenario caused by a behavior of Rail.root with symlink

The failing scenario was caused by a change in Rails that affects
Rails.root when a Rails::Application is defined in symlinked file.

https://github.com/rails/rails/pull/17782

We've been set up Aruba workspace in each Cucumber scenario with
symlinks, but this tends to behave differently from real files.
Since the example_app project is not so large, we can simply clone it in
each scenario.
Remove strange conditional in generate:app task

The `bindir` is rspec-rails' one. We want to run `bundle binstubs` in
the `tmp/example_app` project so the conditional `if test, 'd', bindir`
is useless.
Merge pull request #1256 from rspec/update-travis-build-scripts-2014-12-26-for-master

Updates from rspec-dev (2014-12-26)

[ci skip]
Merge pull request #1257 from rspec/fix-build

Fix Ruby 1.9.3 and Rails 4.1 builds
Force generators to abort if they fail.

Rails does not seem to provide a clean way to abort something if a
generator fails. Under most circumstances this is probably the proper
solution.

However, for us, these generators calls also function as smoke tests for
our actual generator code. If one of them fails, we need to fail the
build. Thankfully, if the generator had a problem the status is captured
in `$?`. We can verify that the last process was successful and abort
when it is not; thus failing the build and notifying the team.
Fix broken generator.

The generator was failing due to the missing require.
Configure Action Mailer `preview_path` in railtie

This mirrors how Rails sets up the default `preview_path`. Using a
`before_initialize` allows the environments to be loaded but shims in
before Rails sets up its `test/` default.
Remove old implementation of setting the path.
Note ActionMailer preview updates in Changelog.
Fix how Rails 4.1 handles showing previews.
Remove skipped spec.

This is being tested via the rake task which uses the generated example
app. It would be nice to have a lighter weight method of testing the
railties in the main RSpec suite, until then keeping this :skull: spec
around just clutters the code base.