Upgrade Aruba for Windows compatibility

I got this error when I tried running the tests on my Windows machine.

```
$ bundle exec rake
c:/Ruby192/bin/ruby -S bundle exec cucumber
c:/Ruby192/lib/ruby/gems/1.9.1/gems/aruba-0.5.1/lib/aruba/api.rb:327: syntax error, unexpected $end, expecting keyword_end
      return eot if "♦" == input ##If on Windows it is actually
                                                                ^ (SyntaxError)
c:/Ruby192/lib/ruby/gems/1.9.1/gems/aruba-0.5.1/lib/aruba/cucumber.rb:1:in `require'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/aruba-0.5.1/lib/aruba/cucumber.rb:1:in `<top (required)>'
c:/Sites/suspenders/features/step_definitions/suspenders_steps.rb:1:in `require'
c:/Sites/suspenders/features/step_definitions/suspenders_steps.rb:1:in `<top (required)>'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/rb_support/rb_language.rb:137:in `load'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/rb_support/rb_language.rb:137:in `load_code_file'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime/support_code.rb:180:in `load_file'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime/support_code.rb:83:in `block in load_files!'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime/support_code.rb:82:in `each'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime/support_code.rb:82:in `load_files!'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime.rb:182:in `load_step_definitions'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/runtime.rb:41:in `run!'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/cli/main.rb:43:in `execute!'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/lib/cucumber/cli/main.rb:20:in `execute'
c:/Ruby192/lib/ruby/gems/1.9.1/gems/cucumber-1.2.3/bin/cucumber:14:in `<top (required)>'
c:/Ruby192/bin/cucumber:19:in `load'
c:/Ruby192/bin/cucumber:19:in `<main>'
rake aborted!
Command failed with status (1): [c:/Ruby192/bin/ruby -S bundle exec cucumbe...]

Tasks: TOP => default => cucumber
(See full trace by running task with --trace)
```

FYI my specs were:

* Windows 7 Home Premium 6.1.7601 Service Pack 1 Build 7601
* Ruby 1.9.2p290

I managed to track down this issue in cucumber/aruba#136, that gets fixed in cucumber/aruba@d5d2517, and then released in Aruba 0.5.2.

The gemspec's `~> 0.5` picks up Aruba 0.5.1, so I upgraded it to 0.5.2 and the error went away.

I also ran the tests on my MacBook and verified that this didn't break anything else.

* Mac OS X 10.7.5
* Ruby 1.9.3p433