S3 signer generates incorrect url for region us-east-1

[louischan-oursky]

Expected Results

The generated URL should always be valid.

https://s3.amazonaws.com/<bucket>/<object-name>

Actual Results

https://s3-us-east-1.amazonaws.com/<bucket>/<object-name>

Steps to reproduce

1. Use a S3 bucket located in region us-east-1
2. ASSET_STORE_PUBLIC=YES
3. skygear.asset.get_signer().sign(some_asset_id)

Cause

When ASSET_STORE_PUBLIC=YES, we naively concat the region to build the URL. The valid format is documented here

Workaround

Use ASSET_STORE_S3_URL_PREFIX=https://s3.amazonaws.com/<bucket>