Examples in readme don't work

[jtippett]

Trying to run the example in the readme:

box = Sodium::SecretBox.new

message = "foobar"
encrypted, nonce = box.encrypt message

# On the other side.
box = Sodium::SecretKey.new key
message = box.decrypt encrypted, nonce: nonce

Gets me this:

In enc.cr:23:7

 23 | box = Sodium::SecretKey.new key
            ^----------------
Error: undefined constant Sodium::SecretKey

I see various other implementations scattered through specs, examples and elsewhere. Is there a definitive version people should be following?