Remove support of redis 2.4

All tests require at least redis 2.6 from now on. Anyone who wants to run the tests should be able to install a newer version.

test/batch.spec.js

@@ -328,8 +328,6 @@ describe("The 'batch' method", function () {
                 });
 
                 it("should work without any callback", function (done) {
-                    helper.serverVersionAtLeast.call(this, client, [2, 6, 5]);
-
                     var batch = client.batch();
                     batch.set("baz", "binary");
                     batch.set("foo", "bar");