Fix non-existent file spec for Ruby 2.1.x.

Ruby 2.1's error messages when faced with non-existent files or directories
have changed compared with earlier versions, as described at
https://bugs.ruby-lang.org/issues/9285

Change the spec to use a regex match, rather than string comparison for
cli spec.

This should fix the failing 2.1.2 Travis test in https://github.com/gds-operations/vcloud-launcher/pull/47