mruby 1.4.1 dispatch breaks go-mruby

[erikh]

while delving into a few bugs I realized that this was a huge issue moving forward, basically the whole notion of a target_class is hidden behind opaque interfaces, which destroys how we couple go and mruby code.

It looks like the solution is just to start using the new proc constructors, but I haven't dug in yet; am not entirely sure when I will have the time to, so recording here.