Use double quotes so #{} works.

lib/geocoder/lookups/maxmind_local.rb

@@ -9,7 +9,7 @@ module Geocoder::Lookup
         gem = RUBY_PLATFORM == 'java' ? 'jgeoip' : 'geoip'
         require gem
       rescue LoadError
-        raise 'Could not load geoip dependency. To use MaxMind Local lookup you must add the #{gem} gem to your Gemfile or have it installed in your system.'
+        raise "Could not load geoip dependency. To use MaxMind Local lookup you must add the #{gem} gem to your Gemfile or have it installed in your system."
       end
       super
     end