This doesn't look right to me, now-where is the variable `composer_version` initialized? Causing an error if the variable is left un-set.

Correct me if I am wrong but assume the user has set composer version

```ruby
set :composer_version, "v1.0.2"
```

Would this not result in the following command being executed

```ruby
execute :curl, "-s", fetch(:composer_download_url), "|", :php, "v1.0.2"
```

I think this is the intended behaviour.

```ruby
composer_version = fetch(:composer_version)
composer_version_option = composer_version ? "-- --version=#{composer_version}" : ""
```