Require openssl

If not required, library will trigger an exception: `uninitialized constant ShippingEasy::Signature::OpenSSL (NameError)` during configuration:

```ruby
require "shipping_easy"

ShippingEasy.configure do |config|
  config.api_key    = "KEY"
  config.api_secret = "SECRET"
end
```

Did not test on all rubies, but this appears on MRI 2.1.2