Fix installation error

[katsusuke]

In the current plugin, the following error occurs during installation with the latest version of heroku cli.

➜  heroku-mongodb git:(master) heroku -v
heroku/7.7.8 darwin-x64 node-v10.7.0
➜  heroku-mongodb git:(master) heroku plugins:link git/github.com/dot/heroku-mongodb                       
(node:33430) Error Plugin: heroku-mongodb: files attribute must be specified in /Users/katsusuke/git/github.com/dot/heroku-mongodb/package.json

I add files attribute to package.json and that error was fixed.

cf heroku/cli#909