(PUP-2710) For audit only, compare Time objects as strings

Previously, auditing the mtime or ctime properties of a file
on ext4 (or any nanosecond resolution filesystem) when running
under ruby 1.9+ would spuriously report a change on every catalog
run.

This was because the yaml library (or libraries, more on that
shortly) correctly serialize the nanosecond resolution of the Time
object to yaml, *but* on deserialization they produce a Time object
truncated to microseconds. This led to a false inequality operation
leading to the spurious audit messages.

This patch is an unfortunate hack consisting of special-casing
the comparison of Time objects in the audit equality operation.

For context, I also considered alternatives:
* Monkey-patching the Time object == method to ignore nanoseconds.
  But this would affect all Time objects and could have undesired
  consequences.
* Monkey-patching the yaml library to correctly deserialize. But
  this got messy because there are several different yaml libraries
  that may be involved.
* Intercepting the serialization in Puppet::Util::Yaml, but this
  would mean discarding nanoseconds for all serialization of Time
  objects to yaml.

In the end, this spot fix seemed like the least evil solution.
Merge pull request #2727 from kylog/issue/pup-2710/audit-comparison-should-compare-strings

(PUP-2710) For audit only, compare Time objects as strings