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.
Merge pull request #48 from bazbremner/fix_non_existent_file_spec_on_ruby_2_1

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