fix: unblock deploys (upgrade ddtrace & config, switch to terser, and enable spec coverage)

[smudge]

Summary

Older versions of the ddtrace gem are now doing some in-place upgrade magic that breaks the datadog.rb initializer as it was written. This change:

• Upgrades to the latest version of ddtrace
• Fixes the initializer to work on that version
• Loads ddtrace (but disables tracing) in tests so that we can detect such breakages going forward

For the sake of unblocking deployments, this also:

• Switches from :uglifier to :terser for ES6 support
• Enables :terser js compression in CI so that we can be alerted to build failures sooner

/domain @Betterment/test_track_core /no-platform