Allow API key setting via ENV variable

Thus we can remove the following section of authy/authy-devise.

> First create an initializer in `config/initializer/authy.rb`

> ```ruby
Authy.api_key = ENV['AUTHY_API_KEY'] || 'your_authy_api_key'
Authy.api_uri = 'https://api.authy.com/'
```