Remove mongo support.

.travis.yml

README.md

db/seed.js

server/config/config.go

server/config/config_test.go

server/store/mongo.go

server/store/store.go

server/store/store_test.go

vendor/gopkg.in/mgo.v2/LICENSE

vendor/gopkg.in/mgo.v2/Makefile

vendor/gopkg.in/mgo.v2/README.md

vendor/gopkg.in/mgo.v2/auth.go

vendor/gopkg.in/mgo.v2/bson/LICENSE

vendor/gopkg.in/mgo.v2/bson/bson.go

vendor/gopkg.in/mgo.v2/bson/decimal.go

vendor/gopkg.in/mgo.v2/bson/decode.go

vendor/gopkg.in/mgo.v2/bson/encode.go

vendor/gopkg.in/mgo.v2/bson/json.go

vendor/gopkg.in/mgo.v2/bulk.go

vendor/gopkg.in/mgo.v2/cluster.go