Update version to 0.6c.

- mesgcomp.py: Fix sent/received: do not try to be clever and expand tuple
  for the case of 1 free variable.
  Originally, in order to support writing

  for c2 in received(Reply(_, c3))

  we expanded the tuple elements of the 'received' set, so that in this case
  it returns

  {c for (c, d) in self.received if d == c3}

  However, if the variable 'c' evaluates to boolean False, then any code
  using 'received' in a boolean context will fail, such as:

  await(any(received(Mesg(_))))

  To fix this we make 'received' always return a set of tuples, regardless of
  the number of free variables it contains.
Merge branch 'master' of github.com:sadboy/DistAlgo