pip-review: fix interactive mode for Python2

With Python2 (2.7.5), `input` exists and is quite different from
`raw_input`:

    Help on built-in function input in module __builtin__:

    input(...)
        input([prompt]) -> value

        Equivalent to eval(raw_input(prompt)).

Therefore, this patch overwrites `input` with `raw_input`, if it exists.

Source: http://stackoverflow.com/a/18332097/15690
Merge pull request #71 from blueyed/fix-input-with-python2

pip-review: fix interactive mode for Python2