Re-enable New Relic transaction tracer

This was disabled in an attempt to fix #60
but the issue still occurred even when New Relic was disabled. This
re-enables the setting per #60 (comment)
and #296 (review)

conf/newrelic.ini

@@ -8,10 +8,6 @@
 app_name = "checkmate"
 monitor_mode = true
 
-# Disable the transaction tracer due to suspected failures caused by cross
-# thread chatter: https://github.com/hypothesis/checkmate/issues/60
-transaction_tracer.enabled = false
-
 # Configuration overrides for specific environments
 
 [newrelic:dev]