Time: Coalesce to Float before Conversion

Time.at can throw an exception if passed a string that is overly precise. Converting to float prior to this avoids that. Note that the epoch may be returned when the string cannot be converted to a float; previously an exception was raised.

Support Mongoid 4

Merge pull request #8 from apptentive/update-for-mongoid-4

Update to support mongoid 4 and bson 2.0 gem. Bump to 1.0 since seria…