Fix :consistency option deprecation in mongodb

consistency is deprecated in mongo

Switch from consistency: :strong to read: :primary
https://github.com/mongoid/mongoid/issues/3429
Ignore swap file
Use geoNear feature of mongodb for randomizing query

Use Array as _randomization_key to store coordinates on [float, 0] space
When retrieving a random document, perform search for closest document by on [float, 0] line
Update random_spec to use expect syntax
Update Guardfile
Clean up Guardfile

Remove rails related path from Guardfile