Add rake-compiler-dock as a dep.
diff --git a/ruby/google-protobuf.gemspec b/ruby/google-protobuf.gemspec
index 312a0c8..0762da9 100644
--- a/ruby/google-protobuf.gemspec
+++ b/ruby/google-protobuf.gemspec
@@ -9,11 +9,12 @@
   s.email       = "protobuf@googlegroups.com"
   s.require_paths = ["lib"]
   s.files       = `git ls-files -z`.split("\x0").find_all{|f| f =~ /lib\/.+\.rb/}
-  unless RUBY_PLATFORM == "java"
+  if RUBY_PLATFORM == "java"
+    s.files     += ["lib/google/protobuf_java.jar"]
+  else
     s.files     += `git ls-files "*.c" "*.h" extconf.rb Makefile`.split
     s.extensions= ["ext/google/protobuf_c/extconf.rb"]
-  else
-    s.files     += ["lib/google/protobuf_java.jar"]
+    s.add_development_dependency "rake-compiler-dock"
   end
   s.test_files  = ["tests/basic.rb",
                   "tests/stress.rb",