Only convert the dashes in version if it exists.

Fixes an issue when defining a dependency, without a version.

```
/usr/share/ruby/find.rb:38:in `block in find': No such file or directory (Errno::ENOENT)
	from /usr/share/ruby/find.rb:38:in `collect!'
	from /usr/share/ruby/find.rb:38:in `find'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/package/dir.rb:79:in `clone'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/package/dir.rb:34:in `block in input'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/package/dir.rb:32:in `chdir'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/package/dir.rb:32:in `input'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/command.rb:321:in `block in execute'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/command.rb:320:in `each'
	from /usr/local/share/gems/gems/fpm-0.4.31/lib/fpm/command.rb:320:in `execute'
	from /usr/local/share/gems/gems/clamp-0.3.1/lib/clamp/command.rb:64:in `run'
	from /usr/local/share/gems/gems/clamp-0.3.1/lib/clamp/command.rb:126:in `run'
	from /usr/local/share/gems/gems/fpm-0.4.31/bin/fpm:8:in `<top (required)>'
	from /usr/local/bin/fpm:23:in `load'
	from /usr/local/bin/fpm:23:in `<main>'
```