Make geokit frozen string friendly

[vvondra]

https://freelancing-gods.com/2017/07/27/friendly-frozen-string-literals.html

Avoids errors like

     Failure/Error: geo_address = Geokit::Geocoders::GoogleGeocoder.reverse_geocode("#{self.latitude},#{self.longitude}")
     
     FrozenError:
       can't modify frozen String

[coveralls]

Coverage remained the same at 96.285% when pulling 125d031 on vvondra:frozen_friendly into 51cfd6c on geokit:master.

[mnoack]

Thanks @vvondra - I don't normally cut releases straight away (I like to let people point to master and test against production for a bit and wait for other changes to bundle into a release)