return nil if array index indicie is out of bounds

ruby arrays don't throw an exception; they return nil.  Lets do the
same!

this fix also includes the ability to use negative array indicies

ruby/README.md

diff --git a/ruby/README.md b/ruby/README.md
index 9ae3ac3..1647432 100644
--- a/ruby/README.md
+++ b/ruby/README.md
@@ -63,7 +63,7 @@
 To Build the JRuby extension, you will need:
 
 * Maven
-* The latest version of the protobuf java library
+* The latest version of the protobuf java library (see ../java/README.md)
 * Install JRuby via rbenv or RVM
 
 First switch to the desired platform with rbenv or RVM.
@@ -75,7 +75,8 @@
 
 Then build the Gem:
 
-    $ rake gem
+    $ rake
+    $ rake clobber_package gem
     $ gem install `ls pkg/google-protobuf-*.gem`
 
 To run the specs: