Update find_and_modify.js

```Meteor.Collection``` is deprecated, replace ```Meteor.Collection``` with ```Mongo.Collection```

Check: https://github.com/meteor/meteor/blob/release/METEOR%400.9.1/History.md#backwards-compatible-renames
Update find_and_modify_tests.js

```Meteor.Collection``` is deprecated, replace ```Meteor.Collection``` with ```Mongo.Collection```

Check: https://github.com/meteor/meteor/blob/release/METEOR%400.9.1/History.md#backwards-compatible-renames
Merge pull request #2 from boustanihani/master

Replace Meteor.Collection with Mongo.Collection