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 binstub rspec` generates a binstub file in the `bin`
directory. Running `./bin/rspec` is now the same thing as running
`bundle exec rspec`.

Adding `export PATH=".git/safe/../../bin:$PATH"` and running `mkdir
.git/safe` in the root of repositories you trust lets us just type
`rspec`, have the binstub be invoked, and therefore the correct version
of RSpec be used for the project.

https://github.com/sstephenson/rbenv/issues/309

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`.