Get environment from RACK_ENV if exists by default

Hi, it may be a good idea to get the environment from RACK_ENV by default. 

We use this in production and recently traced a bug back to the environment not being set in Guard::Puma, so I figured others might have made the same mistake.
I'd prefer to use a `context` block here. Something like:

```ruby
context "when RACK_ENV is set" do
  it "uses the value of RACK_ENV" do
    # ...
  end
end
```