Undefined method capture_message fixed

`sidekiq_retries_exhausted` is not able to invoke normal class method.
As a result "undefined method `capture_message` for
Atmosphere::VmTagsCreatorWorker:Class" was thrown. Fix introduces direct
invocation of `Raven.capture_message`.

To test this behavior I have also upgraded `rspec-sidekiq` into stable
released version 2.0.0

Fixes #11

/cc @tbartynski, @nowakowski 
Prefer single-quoted strings when you don't need string interpolation or special symbols.