Syntax error in Python 2.7

I get a syntax error using old Python 2.7 due to double splat unpacking in _scipy_func at this line:
```
obj = objective_function(*torch_vars, *args)
```
A fix that worked for me was to use functools.partial like so
```
of = partial(objective_function, *args)
obj = of(*torch_vars)
```

Thanks for the very handy library!