Use Resque.validate instead of custom .validate_job!

Job class validation is now a part of resque API.
No need for custom method anymore

Adapted from #181 submitted by @bogdan
More rubocop fixes while we're at it

plus disabling the `ClassLength` cop (for now) :scream_cat:
Merge pull request #270 from meatballhat/using-resque-validate-method

Use Resque.validate instead of custom .validate_job!