Remove gzip_http_version parameter

The `gzip_http_version` override is no longer needed because CloudFront
now forwards requests using HTTP/1.1. We can remove this line entirely
and let nginx use its default value of 1.1.

Ref: [Request and Response Behavior for Custom Origins > HTTP Version](http://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/RequestAndResponseBehaviorCustomOrigin.html#RequestCustomHTTPVersion)