exam:iterate & EMBER_ENV

When running ```ember exam``` it sets the following ember-cli variables as expected:
```
EMBER_CLI_TEST_COMMAND: 'true',
EMBER_ENV: 'test'
```

However, when running ```ember exam:iterate N``` the environment defaults to development
```
EMBER_ENV: 'development'
```

Looks like this line is missing the test environment configuration option:
https://github.com/trentmwillis/ember-exam/blob/master/lib/commands/exam/iterate.js#L77
```js
'./node_modules/.bin/ember build --environment=test --output-path ' + this._outputDir, { stdio: 'inherit' }
```
instead of:
```js
'./node_modules/.bin/ember build --output-path ' + this._outputDir, { stdio: 'inherit' }
```