Fix #366 (Arch Linux `truncate_platform_version': undefined method `split' for nil:NilClass (NoMethodError)).
Update test for platform_version in metadata to check for 'rolling' as /etc/arch-release no longer contains a version hint (deprecated some time ago).
rolling, rolling.
Merge pull request #367 from flaccid/archlinux_platform_version

Fix #366 Arch Linux does not have major and minor release numbers