Allow passing a specific AWS region to S3 settings

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>"
```

config/initializers/db2fog.rb

@@ -7,3 +7,5 @@ DB2Fog.config = {
     :directory             => ENV['S3_BACKUPS_BUCKET'],
     :provider              => 'AWS'
 }
+
+DB2Fog.config[:region] = ENV['S3_REGION'] if ENV['S3_REGION']