geocoder has http proxy support, but it's not obvious from the documentation for where to configure it.
you can find it when looking at the initializer, that should get created for your rails generate call: https://github.com/alexreisner/geocoder/blob/master/lib/generators/geocoder/config/templates/initializer.rb
Geocoder.configure(
[...]
# :http_proxy => nil, # HTTP proxy server (user:pass@host:port)
# :https_proxy => nil, # HTTPS proxy server (user:pass@host:port)
)