Close any files we open in getSourceSchema

I was trying to run a Cascading job on Elastic Map Reduce, reading from many Avro files stored on S3. It kept timing out trying to get an HTTP connection from a connection pool in order to read the schema from the Avro input files. I found that the connections opened in getSourceSchema were never being closed. I made this change and it fixed my issue. Please let me know what I can do to help get this merged into cascading.avro.