Fix bugs in _score() method of Meteor class

[doiken23]

I fixed 2 bugs in _score() function of Meteor class.

1. add .encode() and .decode() to stdiin and stdout functions, respectively.
2. remove overlapped line

for the 2nd bug fix, please see the discussion in https://github.com/jcjohnson/densecap/blob/master/eval/meteor_bridge.py#L42-L46

[jiahuei]

The _score() function is not used by METEOR at all, and can be safely removed.
Thus, the bugs are actually not relevant.