Fix Ruby 2.0.0 warnings

On Ruby 2.0.0, I get warnings as follows:

    /Users/chrishol/code/git-clones/mongoid_denormalize/lib/mongoid_denormalize.rb:106: warning: wrong element type nil at 0 (expected array)
    /Users/chrishol/code/git-clones/mongoid_denormalize/lib/mongoid_denormalize.rb:106: warning: ignoring wrong elements is deprecated, remove them explicitly
    /Users/chrishol/code/git-clones/mongoid_denormalize/lib/mongoid_denormalize.rb:106: warning: this causes ArgumentError in the next release

Compacting the array before doing Hash[...] avoids nils and removes the warnings.