Use Rails-recommended binstub for RSpec

Some Ruby gems provide an executable to run its contained Ruby program. We
most commonly use `rails`, `rspec`, and `rake`.

When we type `rspec` within the root of our project, Rubygems will use the
latest version of the RSpec gem, not the version specified in the project's
`Gemfile`. That's a problem that Bundler's `bundle exec` solves.

It's tedious to type `bundle exec`, however. We previously solved that
problem via a directory convention for Bundler's binstubs and adding that
directory to our shell's `PATH`.

Running `bundle install --binstubs=bin/stubs` generates binstub files in the
`bin/stubs` directory for all the executables specified by gems in our
`Gemfile`. Running `./bin/stubs/rspec` is now the same thing as running
`bundle exec rspec`.

Adding `export PATH="./bin/stubs:$PATH"` lets us just type `rspec`, have the
binstub be invoked, and therefore the correct version of RSpec be used for
the project.

We previously used `./bin/stubs` as our binstubs directory convention and
ignored the directory in version control. We used that convention instead of
`./bin` because we didn't want to gitignore the already-existing `./bin`
directory and we didn't want to replace the critical `bin/rails`.

The community is moving towards a convention of using `./bin`:

* Rails is using `./bin` instead of `./script` starting with Rails 4.
* The default Bundler behavior is to use `./bin`.