Use a set to hold serializable classes

Sets are faster at determining whether a value is included, which is
the only reason this constant exists. We just need to know if the value
is of a type that the database can serialize natively.
Depend on RSpec < 3.0

We use some features of RSpec that were removed in 3.0
Use Time.parse instead of rolling our own

Time.parse understands Postgres-formatted time strings, apparently.