Add an explicit to_i on the ttl value

With the introduction of Rails 4.2, the return value of expressions like `7.days` has changed.  In Rails 4.1.x and below, this is a Fixnum.  In Rails 4.2 and up, it's an ActiveSupport::Duration.

JRuby does not handle converting this new type transparently to a Fixnum during the packing process.  So to eliminate the conversion error we explicitly call to_i.