Fix rails deprecation warnings (use after_commit -> { })

[sergeykish]

Hello

That's part of the rails 5 deprecation warnings cleanup.
It fixes DEPRECATION WARNING: Passing string to define callback.
Change is almost mechanical and I hope new lambda syntax is quite old already (ruby 1.9).

I've also noticed specs failure with rspec 3, changed gemspec to use rspec 2 instead.

[sergeykish]

@lostboy any problem with this pull request?

[unmultimedio]

@sergeykish This repo seems really inactive. Any thoughts on mantaining your own version with Rails 5 deprecation cleanup, and 5.1 support?

[lostboy]

Hey guys, I've left this for so long, apologies for going awol. If people are still keen I can put some time to bringing the repo and gem up to speed.

[unmultimedio]

@lostboy glad you're back. I've checked this in depth couple of months ago, and I've found one fork that has been mantained and updated to work with Rails 5: https://github.com/davidakachaos/workless_revived & It's working ok!

I think you should talk to @davidakachaos to join efforts! Thanks for coming back.