Allow newer versions of gems

We have several clients using our rails application. To not install backup gem on each server we added it to Gemfile (with ```require: false```). But we got a conflict with virtus gem. Virtus depends on axiom-types and axiom-types requires thread_safe to be >= 0.3.1, ~> 0.3 while backup requires thread_safe to be exactly 0.1.3.