Replace sorting plugin with sort keys

[nunofgs]

This PR replaces the sorting-plugin with the sort-keys rule which is available since eslint 3.3.0.

The motivation for this change is to respect the team's natural sort order requirement. Also, it's one less dependency (yay!).

Example that did not trigger an error with sorting-plugin (and was not configurable):

{
  var1: 'foo',
  var10: 'foo',
  var2: 'foo',
  var9: 'bar'
};

Using the new sort-keys rule, the object would have to be written as:

{
  var1: 'foo',
  var2: 'foo',
  var9: 'bar',
  var10: 'foo'
};

😌

[rplopes]

Missing test updates in test/fixtures/correct.js

[rplopes]

WDYT about adding a var1 as well, to test the natural order? Otherwise, both tests could pass with a non-natural descending order.