Ensure Rake::DSL is truly available

The TeamCity CI server runs Rspec tests in an unusual environment. I haven't gotten to the bottom of it, but it somehow creates a Rake class (or constant) but Rake::DSL isn't available. This fails all tests with the following:

```
.../dotenv-0.7.0/lib/dotenv/railtie.rb:5:in `<class:Railtie>': uninitialized constant Rake::DSL (NameError)