Fixed bug where too much cuddling broke Sweetiebot

The `get_next` method in `ResponseFile` doesn't perform bounds checking. This means that if sweetiebot is pinged enough times with `/me` then `sass_index` will go past the array bounds and produce an `IndexError`.

    (5:09) Vinyl Scratch complains to Sweetiebot about ES6
    (5:09) Sweetiebot complains to Vinyl Scratch about Python :sweetie:
    ... to a private chat ...
    (5:12) Vinyl Scratch tests sweetiebot
    (5:12) sweetiebutt@friendshipismagicsquad.com puts Vinyl Scratch into a blender :sweetiesmug:
    (5:12) Vinyl Scratch tests sweetiebot
    (5:12) sweetiebutt@friendshipismagicsquad.com drags Vinyl Scratch off to the cardboard box :sweetiesmug:
    ... SNIP 10 ping/resonse pairs ...
    (5:15) Vinyl Scratch tests sweetiebot more
    (5:15) sweetiebutt@friendshipismagicsquad.com takes the axiom of choice and duplicates Vinyl Scratch :sweetiesmug:
    (5:15) Vinyl Scratch tests sweetiebot more
    (5:15) sweetiebutt@friendshipismagicsquad.com: [IndexError] My code is problematic :sweetieoops:
    ... back to general ...
    (5:16) Vinyl Scratch breaks Sweetiebot
    (5:16) Sweetiebot: [IndexError] My code is problematic :sweetieoops: