Fixes output from generate_symmetric_key_files

When using the Heroku config (ENV vars) for encryption,
generate_symmetric_key_files would output the key as the iv when no iv
was included in the symmetric-encryption.yml file. This change simply
outputs the correct value.
Merge pull request #33 from CheckMateIO/new-iv-generation-output-fix

Fixes output from generate_symmetric_key_files