Fixes an issue with "${rvm_archives_path}/${rvm_ruby_package_file}"
where ${rvm_archives_path} could point to a valid path but
${rvm_ruby_package_file} could be empty, causing a false match. Also use
parens to be a bit more explicit with the logical groupings.
Reworked the bash if line per mpapis' suggestion in PR 1472.
Merge pull request #1472 from cwgem/rubinius-bash-logic-fix

rvm install rbx-head causes file_exists_at_url warning