Switching active_interaction Gem to 1.3.0 branch.

This lets us use their new `interface` filter, which has been in the
works for a couple of months now; analogous to Mutations' `duck` filter.
We'll need that here shortly… after we fix having been bitten (again) by
the book writing code depending on other code that won't exist for a
section or three yet. :-P

RSpec: 42 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 14 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0
Blogs now have entries, using the new Post class.

This is what happens when you try to follow a mockist's TDDing using
classical TDD. Pffft.

RSpec: 50 examples, 0 failures
Script: 0 tests: 0 failures, 0 errors, 0 pending, 0 omitted
RuboCop: 16 files inspected, no offenses detected
RuboCop global overrides in `.rubocop.yml`: 2
RuboCop local overrides in our source/spec files: 0