security_key defaults to nil, not 'MY_SECURITY_KEY'

`nil` is a useful value here, as it generates unsigned "unsafe" URLs suitable for hitting thumbor running in unsafe mode - often the case in development environments etc. Defaulting to `nil` means development environments can run thumbor + thumbor_rails without any shared secret.

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Here's some notes I made in an internal application pull-request leading up to this:

thumbor-rails defaults security key to "MY_SECURITY_KEY":
https://github.com/rafaelcaricio/thumbor_rails/blob/93710ceea2ceace2f19638d8c8481d333126f747/lib/thumbor_rails.rb#L8

It then instantiates Thumbor::CryptoURL with it:
https://github.com/rafaelcaricio/thumbor_rails/blob/93710ceea2ceace2f19638d8c8481d333126f747/lib/thumbor_rails/helpers.rb#L30

CryptoURL wants nil to operate in unsafe mode:
https://github.com/thumbor/ruby-thumbor/blob/65f899df6f33594621e94239dd94806257bcc796/spec/thumbor/crypto_url_spec.rb#L463-L470