ruby: add a new .dev output to ruby derivations

The idea is to bundle ruby, bundler and bundix together. I was
having issues where bundler was installed with ruby 2.3.0 and I wanted to use
ruby 2.0.0.

With this change all the developer has to do is install `ruby_2_0_0.dev`
either in his environment or in a nix-shell.

pkgs/development/interpreters/ruby/dev.nix

@@ -0,0 +1,24 @@
+/* An environment for development that bundles ruby, bundler and bundix
+   together. This avoids version conflicts where each is using a diferent
+   version of each-other.
+*/
+{ buildEnv, ruby, bundler, bundix }:
+let
+  bundler_ = bundler.override {
+    ruby = ruby;
+  };
+  bundix_ = bundix.override {
+    ruby = ruby;
+    bundler = bundler_;
+  };
+in
+buildEnv {
+  name = "${ruby.rubyEngine}-dev-${ruby.version}";
+  paths = [
+    bundix_
+    bundler_
+    ruby
+  ];
+  pathsToLink = [ "/bin" ];
+  ignoreCollisions = true;
+}