Fixes an issue with the sourcemap file and sources property

When transpiling multiple files, the sourcemap `file` and `sources` properties always reference the first transpiled filename.

My configuration:
File `a/b.js`
File `a/c.js`

Grunt config:
```
6to5: {
      dist: {
		files: [{
			expand: true,
			cwd: 'a',
			src: ['**/*.js'],
			dest: 'tmp/'
		}]
	}
}
```

While I'm not entirely sure this is a `grunt-6to5` issue, I tried to reproduce this bug with the `6to5` command-line tool, but it worked perfectly when using the command-line tool.

This pull request modifies the properties, so the reference to filename in the sourcemap is always the filename of the current transpiled file.