Avoid IO errors and output in the specs 

1. Makes sure the second argument to #run is writable.
2. Makes the third argument a null sink in order to avoid output when the runs are aborted.

### Before:

```
~/Projects/spork $ rspec                                                                                                                                                 (master)
.............../Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `write': not opened for writing (IOError)
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `puts'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `puts'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `rescue in block in initialize'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:19:in `block in initialize'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:18:in `fork'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:18:in `initialize'
	from /Users/dasch/Projects/spork/lib/spork/run_strategy/forking.rb:9:in `new'
	from /Users/dasch/Projects/spork/lib/spork/run_strategy/forking.rb:9:in `run'
	from /Users/dasch/Projects/spork/spec/spork/run_strategy/forking_spec.rb:19:in `block (3 levels) in <top (required)>'
./Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `write': not opened for writing (IOError)
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `puts'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `puts'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:27:in `rescue in block in initialize'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:19:in `block in initialize'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:18:in `fork'
	from /Users/dasch/Projects/spork/lib/spork/forker.rb:18:in `initialize'
	from /Users/dasch/Projects/spork/lib/spork/run_strategy/forking.rb:9:in `new'
	from /Users/dasch/Projects/spork/lib/spork/run_strategy/forking.rb:9:in `run'
	from /Users/dasch/Projects/spork/spec/spork/run_strategy/forking_spec.rb:31:in `block (3 levels) in <top (required)>'
....................................

Finished in 1.57 seconds
52 examples, 0 failures
```

### After:
```
~/Projects/spork $ rspec                                                                                                                                       (fix-test-failure)
....................................................

Finished in 1.58 seconds
52 examples, 0 failures
```