Simplify fetching spec group dependencies

Author: deivid-rodriguez

bundler/lib/bundler/resolver/spec_group.rb

@@ -19,9 +19,6 @@ def initialize(specs, relevant_platforms)
         @source = @exemplary_spec.source
 
         @activated_platforms = relevant_platforms
-        @dependencies = Hash.new do |dependencies, platforms|
-          dependencies[platforms] = dependencies_for(platforms)
-        end
         @specs = specs
       end
 
@@ -45,7 +42,7 @@ def to_s
       end
 
       def dependencies_for_activated_platforms
-        @dependencies[activated_platforms]
+        @dependencies_for_activated_platforms ||= dependencies_for(activated_platforms)
       end
 
       def ==(other)