Check sentry-rails before injecting ActiveJob skippable adapters

[st0012]

Fixes #1541

[codecov-commenter]

Codecov Report

Merging #1544 (8ac6ac5) into master (1fd9654) will increase coverage by 0.36%.
The diff coverage is 33.33%.

❗ Current head 8ac6ac5 differs from pull request most recent head f7adafc. Consider uploading reports for the commit f7adafc to get more accurate results

@@            Coverage Diff             @@
##           master    #1544      +/-   ##
==========================================
+ Coverage   97.89%   98.25%   +0.36%     
==========================================
  Files         222      127      -95     
  Lines       10833     6996    -3837     
==========================================
- Hits        10605     6874    -3731     
+ Misses        228      122     -106     

Impacted Files	Coverage Δ
sentry-delayed_job/lib/sentry-delayed_job.rb	73.33% <0.00%> (ø)
sentry-resque/lib/sentry-resque.rb	73.33% <0.00%> (ø)
sentry-sidekiq/lib/sentry-sidekiq.rb	91.30% <100.00%> (ø)
sentry-raven/lib/raven/transports/http.rb
sentry-raven/spec/raven/instance_spec.rb
sentry-raven/spec/raven/linecache_spec.rb
sentry-raven/lib/raven/utils/deep_merge.rb
sentry-raven/spec/raven/client_state_spec.rb
...lib/raven/integrations/rails/controller_methods.rb
sentry-raven/spec/raven/utils/real_ip_spec.rb
... and 88 more

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