Update the Roo gemspec file to include the required dependencies

The install of Roo (gem install roo) des not also install these dependencids,
without which Roo will not run:
1. spreadsheet
2. zip

Please update the Roo Gemspec to include these dependdecies, or at least
point them out in the documentation (RDoc, Roo website, etc.).

Environment:
C:\>gem list roo

*** LOCAL GEMS ***

roo (1.9.2)

C:\>gem env
RubyGems Environment:
  - RUBYGEMS VERSION: 1.7.2
  - RUBY VERSION: 1.8.7 (2011-02-18 patchlevel 334) [i386-mingw32]
  - INSTALLATION DIRECTORY: C:/Ruby/lib/ruby/gems/1.8
  - RUBY EXECUTABLE: C:/Ruby/bin/ruby.exe
  - EXECUTABLE DIRECTORY: C:/Ruby/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86-mingw32
  - GEM PATHS:
     - C:/Ruby/lib/ruby/gems/1.8
     - C:/Documents and Settings/User1/.gem/ruby/1.8
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/