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

* update docs to simplify build steps
* Gemfile.lock seemed to have an older version specified
* do not check in the pkg dir

Change-Id: Idb29077c153530de78ce28c094442aa8f51ddd25

Change-Id: I54df314660cdb861ad8c4da75a08d4cb97faf638

Change-Id: I8c3717f549c9b4e9d07c77ec5875c9cd62b296ac

This adds a Ruby extension in ruby/ that is based on the 'upb' library
(now included as a submodule), and adds support for Ruby code generation
to the protoc compiler.