Specify MIT license in gemspec

Closes #266

Thanks, @bf4!
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!
add rufus scheduler 'every' notice to readme
Merge pull request #272 from Konnigun/Add_rufus_scheduler_info_to_readme

Add rufus scheduler `every` notice to readme
upgraded dependence of resque to support 1.25
Merge pull request #273 from zhu1230/change_dependency_of_resque

upgraded dependence of resque to support 1.25
Merge remote-tracking branch 'upstream/master'