Bypass validations on Tolk::Sync#sync_phrases

When updating the primary_updated flag on secondary phrases for which
the primary phrase has been updated, use a direct-to-DB call to avoid
invoking possibly heavy ActiveRecord validations or triggering an
exeption when the variables used in the phrase change. (fixes #28)
Merge pull request #29 from elDub/bypass_validations_on_sync

Bypass validations on Tolk::Sync#sync_phrases