Bring back core ext string interpolate file referenced by Rails 3.2

We can't just drop the file because Rails 3.2 requires it directly, so
we keep it around even though it is just empty, since we won't support
Ruby 1.8 anymore.

This should allow Rails 3.2 users under Ruby 1.9 to update to the newest
I18n without problem.
Add gemfile for testing with Rails 4.2
Bundle update