Pull request to stop deprecation warnings about combining `let` with `before(:all)`

Changes `before(:all)` to `before(:each)` to stop certain RSpec deprecation warnings explained [here](https://github.com/rspec/rspec-core/pull/823#issuecomment-16590800) about combining `let` with `before(:all)`. RSpec 3.0 promises to [disallow](https://github.com/rspec/rspec-core/pull/823) this combination.

The `let` statements' readability is improved also.

All tests now pass without warnings.

One of the three similar warnings emitted by `rake spec` was:
```
WARNING: let declaration `recipe` accessed in a `before(:all)` hook at:
rails_apps_composer/spec/rails_wizard/recipe_spec.rb:94.
This is deprecated behavior that will not be supported in RSpec 3.

`let` and `subject` declarations are not intended to be called
in a `before(:all)` hook, as they exist to define state that
is reset between each example, while `before(:all)` exists to
define state that is shared across examples in an example group.
```