Implement method for finding current weather by zipcode

[Gilmoursa]

Create method for finding current weather by zipcode.

api.rb

#Zip format : 33704,US
#Usage: OpenWeather::Current.zip('33704,Us')
    def zip(zip, options ={})
      new(options.merge(zip: zip)).retrieve
    end

[manusajith]

@Gilmoursa can you have a look at why the spec is failing.

[Gilmoursa]

I didn't write a spec for the zip method. I'm not sure why the spec is failing in Travis CI. I ran bundle exec rspec in terminal and I have 39 tests passing and 0 failures.

[vanboom]

:+1:

[valerybugakov]

@manusajith is there a way to help with merge of this PR? Would love to have zipcode method