[util.sh#fetch()] Change invalid comment

Returning a default value has been removed in: 5d53601

Changed it to represent the current functionality. Can be tested from the project dir by running:

$ export ruby_install_dir=$(pwd)/share/ruby-install
$ source share/ruby-install/util.sh
$ fetch ruby/dependencies foo
$ fetch ruby/dependencies $package_manager
automake bison openssl readline libyaml gdbm libffi

postmodern / ruby-install

[util.sh#fetch()] Change invalid comment #341