Remove secrets

- Move SECRET_KEY from base.py to local.example.py where it can be customized
- Put a copy in travis.py so that Travis will still work
- Production and Staging already have a mechanism to pull in secrets. Add SECRET_KEY there

After this commit, I will push the secret files to staging via `fab staging upload_secrets:staging.ini` and if successful, will repeat on production

/cc @calebsmith