Requiredness should be after field id.

Tested with 
java -cp ~/.m2/repository/com/facebook/swift/swift2thrift-generator-cli/0.14.0-SNAPSHOT/swift2thrift-generator-cli-0.14.0-SNAPSHOT-standalone.jar:gen-swift/ com.facebook.swift.generator.swift2thrift.Main -package sum -recursive SumService

and it generates correct result:
struct SumRequest {
  1: required i32 num1;
  2: required i32 num2;
}