Merge pull request #17 from mhfs/sidekiq_upgrade

upgrade sidekiq to 2.12.X
Bump ice_cube to ~> 0.11.0
Bump to v0.3.1.
Remove ruby-head from travis.yml.

Apparently, the guys at Travis removed this RVM package:

$ rvm use ruby-head --install --binary --fuzzy
ruby-head is not installed.
Switch to 3-clause BSD license.
Remove C extension (addresses #4 and #5).
Remove 'rake compile' from travis.yml.
Update README. [ci skip]
Bump Sidekiq to ~> 2.13.0.
Ensure redis locks get unlocked.
Remove jruby-19mode from allowed failures in tarvis.yml.
Fix tests to work with recent Sidekiq::Client changes.
Update CHANGELOG.
Bump to v0.3.2.
Rename Sidekiq::Schedulable#tiq to #recurrence.
Fix README. [ci skip]
Bump to v0.3.3.
Add option to disable last and current occurence to be passed to worker