Remove check if mongo gem needs to be added and just add it in the recipes

Running the gem install multiple times will not hurt anything.
Right now checking to see if the recipe is included is a bit backwards and causes issues if the cookbook is being wrapped.
Unless the cookbook is directly in the nodes run_list, it will not show up as included.
Remove check to see if the current recipe is loaded.

Yes, the current recipe is loaded, it's running the code.
Merge pull request #144 from cpuguy83/master

Remove recipe checks for gem install, move gem install to a separate recipe