Keep hierarchy when updating store with use() function

[ernoaapa]

Previously, when store were updated with Provider.use() function it were moved as last in configuration hierarchy. This caused that usually some other configuration were overriding updated configurations and the updates never get applied.
Updated use() function to not remove/add the store, instead just "re-set" the store so the store key keep the same position in the this.stores hash keys.
Closes #176

Example:

file1.json

{
  "candy": {
    "something": "file1"
  }
}

file2.json

{
  "candy": {
    "something": "file2"
  }
}

var nconf = nconf
  .use('file', { file: 'file1.json' })
  .defaults({candy: {something: 'should never get this because read from files'}});

nconf = nconf.use('file', { file: 'file2.json' });

nconf.get('candy:something') == 'file2'
=> false

nconf.get('candy:something')
=> "'should never get this because read from files"

Expect the value be file2 because the file store were updated with use()

[indexzero]

Thank you for your contribution! We ended up implementing this slightly differently, but you are correct that the lexigraphic sort ordering in this.stores was not being respected. See #265