"Danger" bot doesn't need to ask for strict mode for ES Modules

The relevant code in dangerfile:

```
// Ensure the use of 'use strict'; on all files
const noStrictFiles = newJsFiles.filter(filepath => {
  const content = fs.readFileSync(filepath).toString();
  return !includes(content, 'use strict');
});
```

Basically ES modules are strict mode by default, so the check should look for use of "import"/"export" to determine if flagging is required.