Script should respect /usr/bin/env ruby

Otherwise, rvm and rbenv users will be surprised to see the
Apple system ruby is being used.
If lipo -verify_arch fails, script exits non-zero

The previous test against `$?`.success? always eval'ed to true.
Merge pull request #95 from calabash/feature/verify-arches-should-exit-1-on-missing-arches

Verify arches should exit 1 on missing arches