Fixes related to using blitz-ruby on Ruby 2.0

I found a few issues when trying to use blitz-ruby on our project. I have fixed these issues and would be grateful if you would consider adding them to the master version f blitz-ruby  

Update blitz.gemspec to loosen dependencies
A number of dependencies are outdated and cause compatibility issues with the latest gems available. I have changed the gemspec file to loosen the related issues

Replaced 3rd party gem Hexy with Hexdump due to errors in Ruby 2
Hexy fails with the following error
    /lib/hexy.rb:85: invalid multibyte escape: /[\000-\040\177-\377]/ (SyntaxError)
There are many working Hex handling gems in Ruby. I replaced it with one that does work hexdump

Changed AssertionFailedError to work with Rspec
It has be commented before about the problem with Rspec and Blitz both using AssertionFailedError. I have changed the code so that it mirrors Rspec thus avoiding the issue

Changed Gemfile to point to secure Rubygems source
There was a large security breach at Rubygems earlier this year. The solution is to point directly to rubygems.org via https in the gemfile
Updated Rakefile to use RDoc/task

Fixed RUBY_VERSION check to include 2
The check in utils.rb only checked for 1.9 I added 2.x to this