This solves the error showed below when executed `bundle exec rake
db2fog:backup RAILS_ENV=staging` on Katuma staging
```
Excon::Error::BadRequest: Expected(200) <=> Actual(400 Bad Request)
excon.error.response
:body => "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<Error><Code>AuthorizationHeaderMalformed</Code><Message>The authorization header is malformed; the region 'us-east-1' is wrong; expecting 'eu-central-1'</Message><Region>eu-central-1</Region><RequestId>3DE2763883FC601D</RequestId><HostId>1E6UapnblBqVZM0SeeLrdPNDd+VaQ0nxCWwrQ9mi8HjRo2xevAUwtSq5V3fxhsj4Cj9ynnDroco=</HostId></Error>"
```
This ensures that Db2fog always picks up the latest value of the S3_*
env vars and not the one that was persisted last time. Now you can do
things like `S3_BUCKET=xxx bundle exec rake db2fog:backup` if you had
to.
