Don't start Sentry::SendEventJob's transaction

[st0012]

Fixes #1539

[codecov-commenter]

Codecov Report

Merging #1547 (b4c3b05) into master (3ddb146) will increase coverage by 0.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1547      +/-   ##
==========================================
+ Coverage   97.89%   98.28%   +0.38%     
==========================================
  Files         222      127      -95     
  Lines       10838     7002    -3836     
==========================================
- Hits        10610     6882    -3728     
+ Misses        228      120     -108     

Impacted Files	Coverage Δ
sentry-rails/app/jobs/sentry/send_event_job.rb	87.50% <ø> (+9.72%)	⬆️
sentry-rails/lib/sentry/rails/active_job.rb	97.36% <100.00%> (+0.22%)	⬆️
sentry-raven/lib/raven/utils/real_ip.rb
sentry-raven/lib/raven/transports/stdout.rb
...try-raven/lib/raven/utils/exception_cause_chain.rb
...ven/lib/raven/breadcrumbs/active_support_logger.rb
...entry-raven/lib/raven/breadcrumbs/sentry_logger.rb
sentry-raven/lib/raven/interfaces/http.rb
...y-raven/lib/raven/integrations/rails/active_job.rb
...-raven/spec/raven/integrations/rails/event_spec.rb
... and 87 more

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