Fix for Issue-35: Marking View Crash

This PR is a fix for: #35 

The relationship between a Submission model and Result model changed from 'contains' to 'has many' to support remarks. The _rubric_marks_summary_table.html.erb partial was still operating under the assumption that a Submission model contains a single Result object. This was probably left behind in during the merge. The fix provided here uses the latest Result object in the given Submission model to display marks.