(maint) Add rdoc dependency in Gemfile

Without this patch the Puppet spec tests fail to run using the system
ruby included with Enterprise Linux 7.  This is a problem because EL7 is
a supported platform and it is helpful to use the system packages to run
the Puppet spec tests.

The specific issue is the following from a jenkins job that simply runs
`bundle install --path .bundle/gems` and then `bundle exec rspec spec`.

    + bundle exec rspec spec -fd
    /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/activesupport-3.2.19/lib/active_support/dependencies.rb:251:in `require': cannot load such file -- rdoc/rdoc (LoadError)
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/activesupport-3.2.19/lib/active_support/dependencies.rb:251:in `block in require'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/activesupport-3.2.19/lib/active_support/dependencies.rb:236:in `load_dependency'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/activesupport-3.2.19/lib/active_support/dependencies.rb:251:in `require'
            from /var/lib/jenkins/jobs/puppet/workspace/spec/unit/util/rdoc_spec.rb:5:in `<top (required)>'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `block in load_spec_files'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `each'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/configuration.rb:896:in `load_spec_files'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/command_line.rb:22:in `run'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:80:in `run'
            from /var/lib/jenkins/jobs/puppet/workspace/.bundle/gems/ruby/gems/rspec-core-2.14.8/lib/rspec/core/runner.rb:17:in `block in autorun'

This patch addresses the problem by adding 'rdoc' to the set of
dependencies managed by bundler as a pre-requisite step prior to running
the spec tests.