Add explicit requiring Set from standard library

* Due to ActiveSupport modules autoloading Set class is not loaded after require 'active_support/all' therefore require 'hotcell' fails in clean environment

Before:
```
ruby -I /Users/gordon/dev/hotcell/lib lib/hotcell.rb
/Users/gordon/dev/hotcell/lib/hotcell/tong.rb:8:in `block in <module:Mixin>': uninitialized constant Hotcell::Tong::Mixin::Set (NameError)
	from /Users/gordon/.rvm/gems/ruby-2.0.0-p247@hotcell/gems/activesupport-4.0.0/lib/active_support/concern.rb:114:in `class_eval'
	from /Users/gordon/.rvm/gems/ruby-2.0.0-p247@hotcell/gems/activesupport-4.0.0/lib/active_support/concern.rb:114:in `append_features'
	from /Users/gordon/dev/hotcell/lib/hotcell/tong.rb:47:in `include'
	from /Users/gordon/dev/hotcell/lib/hotcell/tong.rb:47:in `<class:Tong>'
	from /Users/gordon/dev/hotcell/lib/hotcell/tong.rb:2:in `<module:Hotcell>'
	from /Users/gordon/dev/hotcell/lib/hotcell/tong.rb:1:in `<top (required)>'
	from /Users/gordon/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:112:in `require'
	from /Users/gordon/.rvm/rubies/ruby-2.0.0-p247/lib/ruby/site_ruby/2.0.0/rubygems/core_ext/kernel_require.rb:112:in `require'
	from lib/hotcell.rb:17:in `<main>'
```

After:
```
ruby -I /Users/gordon/dev/hotcell/lib lib/hotcell.rb
echo $?
0
```