Don't rely on `set` being required implicitly

[bayan]

Many libraries (e.g. rails, rspec, etc) require set, but non rails applications don't by default.

The issue was encountered when I was experimenting with the library via a fresh ruby console (i.e. pry or irb).

$ irb
irb(main):001:0> require 'futuroscope'
=> true
irb(main):002:0> x = Futuroscope::Future.new{ sleep(1); 1 }
NameError: uninitialized constant Futuroscope::Pool::Set
    from /Users/bayan/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/futuroscope-0.1.5/lib/futuroscope/pool.rb:19:in `initialize'
    from /Users/bayan/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/futuroscope-0.1.5/lib/futuroscope.rb:11:in `new'
    from /Users/bayan/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/futuroscope-0.1.5/lib/futuroscope.rb:11:in `default_pool'
    from /Users/bayan/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/futuroscope-0.1.5/lib/futuroscope/future.rb:29:in `initialize'
    from (irb):2:in `new'
    from (irb):2
    from /Users/bayan/.rbenv/versions/2.0.0-p247/bin/irb:12:in `<main>'
irb(main):003:0> 

[coveralls]

Coverage increased (+0.8%) when pulling 03f7a095fbd8a12478f9ab4162acdb4f94b4ea6e on bayan:master into acb9f95bfc0c823b4018a51a25b7e78f5cf5b6c9 on codegram:master.

[txus]

Good catch, thank you!