You're misunderstanding the gemspec dependencies a bit.
add_development_dependency
is used for adding dependencies that aid in coding of the gem. Examples would be things like TestUnit, Pry, or RSpec. These dependencies won't be available in applications that use your gem.
add_dependency
is used for dependencies which are important for the operation of your gem. For example, geocoder in your case.
If you make that change, you should be good to go.