Use shared mongoose

Use shared mongoose instance, require('gitter-web-mongoose-bluebird') for what we mongoose.connect and mongoose.connection.once('open', ...)

I'm not sure why this suddenly stopped working but the app no longer starts unless I make this change as we wait for mongo in several places.

• mongoose@4.6.8
• @gitterhq/env@0.33.0

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mongoose.connection.once('open', ...)

• modules/persistence-utils/lib/on-mongo-connect.js doesn't fire. Uses require('mongoose')
• @gitterHQ/env/lib/mongoose-connection-configurer.js event fires. Passed in require('gitter-web-mongoose-bluebird') and is where we mongoose.connect