Suggesting minor correction to the source code

[itlchriss]

As the pyyaml has updated, the load function requires a Loader specified.
In the scripts/semantic_index.py, at line 140, the use of load is simply loading a simple yaml.
As the document suggested, the line

loaded = yaml.load(infile)

can be replaced with

loaded = yaml.safe_load(infile)

to avoid the following error

Traceback (most recent call last):
  File "../scripts/semparse.py", line 197, in <module>
    main()
  File "../scripts/semparse.py", line 82, in main
    SEMANTIC_INDEX = SemanticIndex(ARGS.templates)
  File "ccg2lambda/scripts/semantic_index.py", line 32, in __init__
    self.rules = load_semantic_rules(contents)
  File "ccg2lambda/scripts/semantic_index.py", line 140, in load_semantic_rules
    loaded = yaml.load(infile)
TypeError: load() missing 1 required positional argument: 'Loader'

Tested with successfully running the tool.