Extend cache on database failure

[angelim]

Allows consumers to reuse(and extend) an old cache when the database is not available for a refresh. If the cache is not yet bootstrapped we still raise the exception.

This feature is opt-in and managed by Arturo::Feature.extend_cache_on_failure config.

Logging(optional) was added to help debug cache extension events. You'll be able to see one of:

• [Arturo][extend_cache_on_failure] Fallback cache is empty
• [Arturo][extend_cache_on_failure] Falling back to stale cache

Side note:

Also had to lock sqlite3 on older rails gemfiles to make travis happy.

# Gem::LoadError:
#   can't activate sqlite3 (~> 1.3.5), already activated sqlite3-1.4.2. Make sure all dependencies are added to Gemfile.
#   ./spec/support/prepare_database.rb:4:in `<top (required)>'

[angelim]

@craig-day need another review. Had to introduce logging here.