added rules 3.1.14 and 3.1.15

[cezarywojcik]

See diff for new rules.

Preferring static to class for several reasons:

1. Static dispatch is technically faster than dynamic dispatch
2. Given the above, it makes sense to pick static over class so that we consistently use one over the other.
3. This way, usage of class will indicate that the intent of that function is to be overridden, which can be useful information at-a-glance.