Rely on Sidekiq's String#constantize extension instead of rolling our own

As per discussion on issue #17, I've dropped the custom implementation of 
String.constantize in favour of using the built-in Sidekiq implementation.

Currently this helper method behaves the same as it did previously, if the
string could not be constantized, no exception is raised, and the original
string argument is returned.

Note that calling `klass.get_sidekiq_options` on the next line of `call` is 
going to raise an exception anyway if `klass` is a string.