(maint) Fix can't modify frozen Symbol error on Ruby 2.1.0

Without this patch we're getting the following error on Ruby 2.1.0:
`can't modify frozen Symbol (Puppet::Error)`.  This problem is caused by
our monkey patching which attempts to cache Symbol#to_proc return values
instead of creating new instances on every call.

As it turns out, the caching optimization is not necessary on Ruby 1.9.3
and later because Symbol#to_proc implements its own caching system in
these versions.

Here is the full error this patch avoids by not monkey patching
Symbol#to_proc on versions of Ruby prior to 1.9.3.

    bundle exec rspec -fd spec/integration/agent/logging_spec.rb
    /workspace/serious/src/puppet/lib/puppet/util/monkey_patches.rb:79:in `to_proc': Could not autoload puppet/type/component: can't modify frozen Symbol (Puppet::Error)
      from /workspace/serious/src/puppet/lib/puppet/transaction/event.rb:15:in `<class:Event>'
      from /workspace/serious/src/puppet/lib/puppet/transaction/event.rb:8:in `<top (required)>'
      from /workspace/serious/src/puppet/lib/puppet/transaction.rb:13:in `require'
      from /workspace/serious/src/puppet/lib/puppet/transaction.rb:13:in `<class:Transaction>'
      from /workspace/serious/src/puppet/lib/puppet/transaction.rb:11:in `<top (required)>'
      from /workspace/serious/src/puppet/lib/puppet/type/component.rb:4:in `require'
      from /workspace/serious/src/puppet/lib/puppet/type/component.rb:4:in `<top (required)>'
      from /workspace/serious/src/puppet/lib/puppet/util/autoload.rb:61:in `load'
      from /workspace/serious/src/puppet/lib/puppet/util/autoload.rb:61:in `load_file'
      from /workspace/serious/src/puppet/lib/puppet/util/autoload.rb:202:in `load'
      from /workspace/serious/src/puppet/lib/puppet/metatype/manager.rb:159:in `type'
      from /workspace/serious/src/puppet/lib/puppet/type.rb:2466:in `<top (required)>'
      from /workspace/serious/src/puppet/lib/puppet.rb:175:in `require'
      from /workspace/serious/src/puppet/lib/puppet.rb:175:in `<top (required)>'
      from /workspace/serious/src/puppet/spec/spec_helper.rb:16:in `require'
      from /workspace/serious/src/puppet/spec/spec_helper.rb:16:in `<top (required)>'
      from /workspace/serious/src/puppet/spec/integration/agent/logging_spec.rb:2:in `require'
      from /workspace/serious/src/puppet/spec/integration/agent/logging_spec.rb:2:in `<top (required)>'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `load'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `block in load_spec_files'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `map'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/configuration.rb:780:in `load_spec_files'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/command_line.rb:22:in `run'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:69:in `run'
      from /workspace/serious/src/puppet/.bundle/gems/ruby/2.1.0/gems/rspec-core-2.11.1/lib/rspec/core/runner.rb:8:in `block in autorun'
(maint) Fix a comment.
(maint) Fix rdoc generator for Ruby 2.1.0

Ruby 2.1.0 includes RDoc 4.1.0 which added a requirement that
subclasses of ClassModule implement 'add_prefix'. This requirement
was added in this commit:

https://github.com/rdoc/rdoc/commit/1701151b5b14cd20d624c4ce75fd61cc60cd07f3

The rdoc docs for other classes mention that aref_prefix should be
implemented by subclasses, but generally a base class implementation is
provided.  The rdoc doc for ClassModule hasn't been updated to reflect
the new requirement.

This patch simply implements the 'aref_prefix' method.