use SecureRandom to generate pipeline unique ids

This avoids depending on the uuidtools gem, instead favoring Ruby's
stdlib.  Additionally, this causes us to pass a string into Fog, instead
of the UUID object returned by UUIDTools.  This fixes a bug that arises
when active_support/core_ext is loaded, overriding fog-json's #to_json
implementation for many objects, including UUID.

Repro: https://gist.github.com/409287dfdff6355a8a99
Merge pull request #34 from swipely/fix_uuid_serialization

use SecureRandom to generate pipeline unique ids