Update purescript-unfoldable dependency

Fixes the following error for me, which started after upgrading to purescript 0.6.2

    Multiple errors:

    Error at /Users/ryanartecona/Code/PureScript/purescript-markdown/bower_components/purescript-unfoldable/src/Data/Unfoldable.purs line 11, column 1 - line 16, column 1:
    Error in module Data.Unfoldable
    Unknown type 'STArray'

    Error at /Users/ryanartecona/Code/PureScript/purescript-markdown/bower_components/purescript-unfoldable/src/Data/Unfoldable.purs line 17, column 26 - line 31, column 16:
    Error in module Data.Unfoldable
    Unknown value 'runSTArray'