Extra `/` causes the S3 bucket to have an empty "folder" at the beginning

[ibiris]

https://github.com/better/jsonschema2db/blob/60832f5f38d6a257920508c09fec3a65daed4e4c/jsonschema2db.py#L365

If every key string in S3 should start with a / then the path where this key is appended should not have a / otherwise you will end up with paths like

s3://bucketname//prefix/...

[erikbern]

feel free to submit a pull request!