Added diff-cover as a requirement (installed from github)

rakefiles/tests.rake

@@ -46,7 +46,8 @@ directory REPORT_DIR
 
 task :clean_test_files do
 
-    # Delete all non-folder files in the reports directory
+    # Delete all files in the reports directory, while preserving
+    # the directory structure.
     sh("find #{REPORT_DIR} -type f -print0 | xargs -0 rm")
 
     # Reset the test fixtures

requirements/edx/github.txt

@@ -10,3 +10,4 @@
 # Our libraries:
 -e git+https://github.com/edx/XBlock.git@2144a25d#egg=XBlock
 -e git+https://github.com/edx/codejail.git@5fb5fa0#egg=codejail
+-e git+https://github.com/edx/diff-cover.git@v0.1#egg=diff_cover