Issues with environment variables in google_books

We keep running into a weird problem with environment variables. The google books api key is stored on GOOGLE_BOOKS_KEY and accessed via
```elixir
@key System.get_env("GOOGLE_BOOKS_KEY")
```
in google_books.ex. Weirdly the value is only recognised some of the time, so sometimes when @ZooeyMiller or I run the server we get errors due to the key being set to "". If we run `System.get_env` directly in a function instead of through `@key` it loads fine.

As a temporary solution we've moved `System.get_env` to the `create_url` function instead, and set envy up so it only loads `.env`s when not in Mix env test. This may not work in the long-term because sometimes tests need access to environment variables (for example when we add OAuth).