Ruby 1.9 Compatibility: Add missing require

[kmcclellan]

Fixes a compatibility issue introduced by 96a6d94. Monitor needs to be explicitly required in Ruby 1.9.

Steps to reproduce:

$ sudo gem install ref -v 2.0.0
$ irb -r ref

/var/lib/gems/1.9.1/gems/ref-2.0.0/lib/ref/soft_reference.rb:26:in `<class:SoftReference>': uninitialized constant Ref::SoftReference::Monitor (NameError)
	from /var/lib/gems/1.9.1/gems/ref-2.0.0/lib/ref/soft_reference.rb:19:in `<module:Ref>'
	from /var/lib/gems/1.9.1/gems/ref-2.0.0/lib/ref/soft_reference.rb:1:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
	from /var/lib/gems/1.9.1/gems/ref-2.0.0/lib/ref.rb:19:in `<module:Ref>'
	from /var/lib/gems/1.9.1/gems/ref-2.0.0/lib/ref.rb:1:in `<top (required)>'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:60:in `require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:60:in `rescue in require'
	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:35:in `require'
	from /usr/lib/ruby/1.9.1/irb/init.rb:281:in `block in load_modules'
	from /usr/lib/ruby/1.9.1/irb/init.rb:279:in `each'
	from /usr/lib/ruby/1.9.1/irb/init.rb:279:in `load_modules'
	from /usr/lib/ruby/1.9.1/irb/init.rb:20:in `setup'
	from /usr/lib/ruby/1.9.1/irb.rb:53:in `start'
	from /usr/bin/irb:12:in `<main>'

[kmcclellan]

For some additional context, monitor gets loaded by bundler, so this issue doesn't crop up in contexts that use bundle commands (e.g. bundle exec rake by the CI).