Use config transforms on dyno prefixes

The list of Heroku dyno prefixes to shorten was previously hand-rolling
the code to process a config value with a CSV list in it. With the new
config transforms that isn't necessary anymore.

This also makes a slight change to DefaultSource#convert_to_list so it
will raise explicitly on finding a value it couldn't make a list from.
This is caught by the usage upstream in the config manager, and gives
the expected errors in the log for badly formatted config values.
Added CustomEventAggregator, for custom Insights events.
Extend ActiveRecord multiverse to other versions

We were only running the (currently very slim) ActiveRecord multiverse
suite against Rails 3.2. Not much point in that level of coverage.

In the future it'd be nice to see the unit tests against ActiveRecord
move over into this suite now that it covers all the versions.

Additionally this sets things up for creating a multiverse suite against
DelayedJob using the ActiveRecord adapter.
RUBY-1389 Working DJ sampler test

Only against DJ 4.0 + ActiveRecord 3.2 currently
RUBY-1389 Use DJ's migration directly
RUBY-1387 Unified interface of SizedBuffer and SampleBuffer.
Extracted base class EventBuffer from SizedBuffer and SampleBuffer, and
extracted common tests into EventBufferTestCases.
RUBY-1389 Run on most of the DJ versions we support
RUBY-1389 Proper queries on ActiveRecord 4.1
RUBY-1389 Avoid deprecation warnings on AR 4.0
RUBY-1389 Behave gracefully on unsupported DJ backend
RUBY-1389: Add license header.
RUBY-1373 Lock around references to @pipes to prevent race condition.
RUBY-1389 Less alarming warning on unsupported backend
Merge pull request #827 from ruby-agent/pipe_channel_listener_lock

Lock around references to @pipes to prevent race condition.
RUBY-1389 Include DJ in background multiverse group
Merge pull request #828 from ruby-agent/RUBY-1389-dj-sampler

RUBY-1389 DJ Sampler multiverse tests + DJ ActiveRecord 4.1 fix
Skip DJ tests on Ruby 2.2.0-preview1

Known issue that will be fixed with next 2.2.0 version.
Merge pull request #832 from ruby-agent/skip_dj_on_22_for_now

Skip DJ tests on Ruby 2.2.0-preview1
Fix hang on listener tests in Rubinius

Weren't clearing out the listener fully between tests, which ended up hanging.
Merge pull request #833 from ruby-agent/fix_rbx_hang

Fix hang on listener tests in Rubinius