setting DATABASE_URL doesn't ignore defaultEnv from .db-migraterc

## I'm submitting a...

- [x] Bug report
- [ ] Feature request
- [ ] Question

## Current behavior
running `db-migrate` with `DATABASE_URL` defined does not ignore `defaultEnv` from `.db-migraterc`

## Expected behavior
running `db-migrate` with `DATABASE_URL` respects that value and doesn't try to read `defaultEnv` from `.db-migraterc`

## Minimal reproduction of the problem with instructions
1. `export DATABASE_URL=mysql://`
1. create a `.db-migraterc` with a `defaultEnv` configured. I chose local instead of dev for my example.
1. Run `db-migrate`.
1. You will see something like "environment 'local' is not defined"

## What is the motivation / use case for changing the behavior?
When setting database URL, the intent is that it is defining everything about your environment. To respect the `defaultEnv` setting from `.db-migraterc` seems to contradict that

## Environment

db-migrate version: 0.11.1

plugins with versions: unknown, don't think I'm using plugins?

db-migrate driver with versions: 

mysql 1.1.10


Additional information:

- Node version: 8.9.4

- Platform:  MacOS 10.13

Others:

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