And regarding the expection that this would install `6.1.0`:

    composer global require drush/drush:6

It seems that composer's current behavior is on par with `gem`, and perhaps `pear` was the outlier:

```
➜  gem list -ra gherkin-ruby

*** REMOTE GEMS ***

gherkin-ruby (0.3.1, 0.3.0, 0.2.1, 0.2.0, 0.1.0, 0.0.2, 0.0.1)
➜  gem install gherkin-ruby -v 0.2
Fetching: rexical-1.0.5.gem (100%)
Fetching: gherkin-ruby-0.2.0.gem (100%)
Successfully installed rexical-1.0.5
Successfully installed gherkin-ruby-0.2.0
2 gems installed
```

Note that `gherkin-ruby-0.2.1` was not installed.