Explicitly list bindir and executables

Users installing via gem won't have the git history.
Therefore, git-ls won't return anything and the executables won't be available.

Background:

I did `bundle install` but the `magick_collage` and other executables weren't available.

Turns out that the command for finding the executables:
    s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
 
... doesn't work for me because installing from bundler doesn't download the git history.