[rubygems/rubygems] Remove check only necessary for ancient RubyGems

https://github.com/rubygems/rubygems/commit/ffa2f03489

lib/bundler/rubygems_integration.rb

@@ -161,7 +161,7 @@ module Bundler
     def spec_cache_dirs
       @spec_cache_dirs ||= begin
         dirs = gem_path.map {|dir| File.join(dir, "specifications") }
-        dirs << Gem.spec_cache_dir if Gem.respond_to?(:spec_cache_dir) # Not in RubyGems 2.0.3 or earlier
+        dirs << Gem.spec_cache_dir
         dirs.uniq.select {|dir| File.directory? dir }
       end
     end