RUBY-1255 Add multiverse tests for timestamps submitted with metric_data
RUBY-1255 Add a created_at timestamp to StatsHash instances
RUBY-1255 Use created_at timestamp on StatsHash for metric_data begin time

This is instead of tracking the last metric harvest time in the NewRelicService
instance, because the NewRelicService instance will be inherited from the parent
process.
RUBY-1255 unit test for service drawing from stats hash timestamp
RUBY-1255 Rename StatsHash#created_at to started_at
RUBY-1255 Don't allow errors filling the metric_id cache to count as failures
Merge pull request #350 from newrelic/RUBY-1255

RUBY-1255 Fix initial metic_data timestamps when a new worker is forked off
Use json instead of rubysl-json (which has been yanked)

Per brixen's comment here:
https://github.com/bundler/bundler/issues/2774#issuecomment-31967820
Don't try to run unit tests in an environment if bundling there fails
Remove incorrect conditionals around json gem in HTTP client Envfiles
Require at least JSON 1.8.1 in Datamapper suite (rbx breaks with 1.8.0)
Make thread profiling multiverse test less flaky

Two issues being addressed here:

1. There was a race in the way that the test_thread_profiling and
   test_thread_profiling_with_pruby_marshaller tests were set up. We were
   starting the thread profiler and *then* starting the two threads it was
   supposed to take samples of, and then waiting for at least 10 poll cycles,
   and asserting that we saw 10 samples. If the threads didn't have a chance to
   start up before the first few poll cycles, this wouldn't work.

   We now start the threads to be profiled before starting the thread profiler,
   and ensure that they've actually executed the block passed to run_thread
   before proceeding.

2. The profiled threads would sleep for 10s, and then exit. If the test took
   longer than 10s, this wouldn't work. Now they will sleep indefinitely, until
   explicitly woken from the call to Thread#run from join_background_threads.
RUBY-1255 CHANGELOG entry for metric timestamps fix
Merge pull request #352 from newrelic/RUBY-1255-changelog

RUBY-1255 CHANGELOG entry for metric timestamps fix