fix location of msmarco source files and bump version (#257)

Author: seanmacavaney

ir_datasets/__init__.py

@@ -101,4 +101,4 @@ def main_cli():
     import sys
     main(sys.argv[1:])
 
-__version__ = "0.5.5" # NOTE: keep this in sync with setup.py
+__version__ = "0.5.6" # NOTE: keep this in sync with setup.py