rename movist-fork.rb to movist.rb

There is no need to maintain this under a name which is
different from the app (Movist.app)  We do not offer a
separate "movist" Cask, and the original project (from
which this forked) is dead.
Merge pull request #3220 from rolandwalker/movist_rename

rename movist-fork.rb to movist.rb