Update ruby-public_suffix to 3.0.0.

#### Release 3.0.0

This new version includes a major redesign of the library internals, with the
goal to drastically improve the lookup time while reducing storage space.

For this reason, several public methods that are no longer applicable have
been deprecated and/or removed.  You can find more information at GH-133.

- CHANGED: Updated definitions.
- CHANGED: Dropped support for Ruby < 2.1
- CHANGED: `PublicSuffix::List#rules` is now protected. You should not rely on
  it as the internal rule representation is subject to change to optimize
  performances.
- CHANGED: Removed `PublicSuffix::List.clear`, it was an unnecessary accessor
  method. Use `PublicSuffix::List.default = nil` if you **really** need to
  reset the default list. You shouldn't.
- CHANGED: `PublicSuffix::List#select` is now private. You should not use it,
  instead use `PublicSuffix::List#find`.
- CHANGED: `PublicSuffix::List` no longer implements Enumerable. Instead, use
  `#each` to loop over, or get an Enumerator.
- CHANGED: Redesigned internal list storage and lookup algorithm to achieve
  O(1) lookup time (see GH-133).

(taca)