Use blocked scoped variables

In addition to using const and let, this also upgrades our linter config and removes lint (mostly whitespace).

test/spec/ol/control/rotate.test.js

@@ -5,7 +5,7 @@ describe('ol.control.Rotate', function() {
   describe('constructor', function() {
 
     it('can be constructed without arguments', function() {
-      var instance = new Rotate();
+      const instance = new Rotate();
       expect(instance).to.be.an(Rotate);
     });