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
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 #1257 from rspec/fix-build

Fix Ruby 1.9.3 and Rails 4.1 builds
Update supported RSpec versions.

Rails 4.2.0 has been officially released. Drop usage of the RC line and
use the main releases.

Rails `master` is now targeting Rails 5.x. Plans for Rails 5.x support
with RSpec is not official yet. To prevent the build from failing too
often, drop support for Rails `master` on rspec-rails `master`.