Adds gem iconv as a dependency for gem Goldencobra.

Iconv is used inside the model Goldencobra::Import. It is not a usually
installed gem and it wasn't listed in the dependencys. One couldn't
load the environment without this gem.
Merge pull request #18 from 5minpause/add-iconv

Adds gem iconv as a dependency for gem Goldencobra.