Measuring latency fails when the resourceURLs have non-standard ports

Currently running `wbench http://localhost:3000` would cause an `Errno::ECONNREFUSED` error on the second iteration.

```
/Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:15:in `initialize': Connection refused - connect(2) (Errno::ECONNREFUSED)
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:15:in `new'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:15:in `block in latency_for'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/2.0.0/benchmark.rb:281:in `measure'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:15:in `latency_for'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:9:in `block in result'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:9:in `map'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/timings/latency.rb:9:in `result'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:36:in `latency_results'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:20:in `block (3 levels) in run'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/browser.rb:26:in `visit'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:20:in `block (2 levels) in run'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:18:in `times'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:18:in `block in run'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:17:in `tap'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:17:in `run'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/lib/wbench/benchmark.rb:4:in `run'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/lib/ruby/gems/2.0.0/gems/wbench-0.3.7/bin/wbench:32:in `<top (required)>'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/bin/wbench:23:in `load'
	from /Users/ronny/.rbenv/versions/2.0.0-p247/bin/wbench:23:in `<main>'
```