Sort revisions in rollout history as integers

Previously keys were sorted as strings, thus it was possible
to see such order as 1, 10, 2, 3, 4, 5.

Ints64 helper implemented in util/slice module to sort []int64

pkg/util/slice/slice_test.go

@@ -68,3 +68,12 @@ func TestShuffleStrings(t *testing.T) {
 		}
 	}
 }
+
+func TestSortInts64(t *testing.T) {
+	src := []int64{10, 1, 2, 3, 4, 5, 6}
+	expected := []int64{1, 2, 3, 4, 5, 6, 10}
+	SortInts64(src)
+	if !reflect.DeepEqual(src, expected) {
+		t.Errorf("func Ints64 didnt sort correctly, %v !- %v", src, expected)
+	}
+}