ensure that S3 Push does not double-slash at beginning

[deitch]

When passing the target key to S3 via the library as:

	_, err = uploader.Upload(context.TODO(), &s3.PutObjectInput{
		Bucket: aws.String(bucket),
		Key:    aws.String(path.Join(key, target)),
		Body:   f,
	})

It expects the Key to start without a slash, as it always adds one. This change ensures that it always is clean.