Default environment should not be dev

I don't think the environment should default to dev because you can accidently populate your dev db if you forget to specify the env paramater.

An example use case is we have 3 servers - dev, int, and prod, which have those same NODE_ENV's set (so the NODE_ENV=dev on the dev server etc.).

All the developers also have their own local mongo instances installed as well. If I just run `$ db-migrate up` on my local machine, it's running those migrations against the dev server even though I just want them to run locally.

I'm advocating that the default environment should be "undefined" because that is what the NODE_ENV is by default.

Then in my `database.json` file i would just have a block for the "undefined" environment which is actually my local environment.

``` json
{
"undefined": {
"driver": "mongodb",
"host": "localhost",
"database": "my_db"
},
}
```

Thoughts?
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