Updated `unique_job` method signature for Ruby 2.x

Updated the definition of unique_job as per @nevans request in #3

This allows one to configure unique job using the default mode and job specific options:

unique_job timeout: 2.hours

Whereas previously you would have been required to provide the first conditional argument mode regardless of whether or not you were using the default value in order to set the options:

unique_job :until_executed, timeout: 2.hours